Sorry; I accidentally omitted a change description. I am in the process of pulling the Walpole Island First Nation boundary off of the International Border. This is work in progress.

Ky_AT, you fixed the issue in changeset 156567714 at 07:36 Eastern Daylight Time (almost 6 hours ago). Thank you for that. I am not sure there is really any more analysis to do. Everything is fine now.

Ky_AT, you broke Lake Saint Clair with this edit.
You added osm.org/way/945926982#map=15/42.57549/-82.61028&layers=ND with an inner role to the Lake Saint Clair Relation. It shares nodes with one of the outer ways. JOSM error is "Multipolygon outer way shares segment with other ring".

Let me know if this breaks anything!

The boundary of Carcross I.R. No. 4 was first surveyed and demarcated in 1959. Survey record is "CLSR 50051 YT".
Carcross I.R. No. 4 still exists today (less the portion carved out by the highway), but it is now a "retained reserve" that is part of the Carcross/Tagish First Nation (CTFN) settlement lands.
The boundary shown by OSM was wrong; it had been copied from Statistics Canada's census subdivision "Carcross 4" which is a crude approximation of the original I.R. boundary. The result of all this is that the map was showing the Carcross I.R. No. 4 aboriginal_lands boundary crossing the CTFN aborignal_lands boundary. It looked like somebody had scribbled all over the map. Even if it had been mapped properly, we would still have had aborignal_lands boundaries nested inside other aborignal_lands boundaries with some overlapping edges.
In order to make the map readable, I have taken the bold step of deleting Carcross I.R. No. 4.
The census subdivision boundary (Carcross 4) will remain in OSM, but it is not normally rendered.
Also note there was a change to Carcross 4 in Statistics Canada's 2021 boundary maps; the boundary now stops at the shoreline of Nares Lake. I have updated the Carcross 4 boundary in OSM accordingly.

In 1984 the Inuvialuit Final Agreement adopted the intersection of the northerly bank of the Kugalak River and the shoreline of Penny Bay in its definition of the Holman 7(1)(b) parcel at the western end of the Wollaston Peninsula on Victoria Island.
The Nunavut Act of 1993 adopted part of the Holman 7(1)(b) boundary as part of western border of Nunavut.
Thus the Nunavut Act includes the words: "thence easterly in a straight line to the intersection of the northerly bank of Kugalak River and the shoreline of Penny Bay in Amundsen Gulf".

Between 1984 and 2015, erosion caused the mouth of the river to move about 1500m south; this is evident when comparing old and new satellite imagery.
In 2015 the federal government paid for a legal survey of the riverbank. Searches show this was the first legal survey that includes the intersection of the river and the shoreline.
The resulting survey plan (105816 CLSR NT) includes a small arrow and the words: "INTERSECTION OF THE NORTHERLY BANK OF KUGALUK RIVER AND THE SHORELINE (OHWM) of PENNY BAY".
The survey includes 36 GPS readings along the northern bank of the river, but unfortunately the spot marked by the arrow was determined from Satellite imagery instead of GPS; so no precise coordinates are provided for the intersection.

Based on features shown on the survey plan, it is possible to determine the location of the arrow with an accuracy of about 10m. If we believe current ERSI imagery, the spot marked by the arrow is now under water due to erosion.

It is not clear if the current boundary should be based on the 2015 survey or if it should move with the river. I expect it would take a team of lawyers to figure that out.
Also keep in mind the river will overflow its banks at certain times of the year causing parts of the coast and the islands in Penny Bay to become submerged.

In this changeset I have redrawn the entire marine boundary from mainland Canada to the Kugalak River. The eastern termination point is within about 20m of the spot marked by the arrow on the 2015 survey plan. The nodes are plotted on a geodesic line which gives the marine boundary a slight curve when viewed on a Mercator map.

The areas of natural=reef in the vicinity of Bove Island were originally added by Ben&Sarah as part of changeset 84304324. I have removed them because I am not convinced the "reef" is the best term to describe an area of shallow water in a fresh water lake. Also, areas of shallow water in fresh water lakes are not normally mapped in OSM so I don't see the point of making a special exception for the shallow water near Bove Island.

The western end of the BC/YT border has never been surveyed and therefore there is no survey monument at the BC/YT/AK tripoint.
The tripoint by definition is the intersection of 60°N(NAD27) and the international border. The relevant part of the international border is the geodesic line between monuments BP177 and BP178.
Previously OSM showed the BC/YT/AK tripoint coinciding with BP177 which resulted in an error of about 685m.

Actually the removed way was called "old Dehcho - South Slave boundary".

Thank you for pointing out the stray colon. I have removed it from all the survey monuments that I added over the past few days.

In 2014 the Survey General Branch of Natural Resources Canada decided that the Northwest Territories / Nunavut boundary should commence at survey monument 157. All the monuments between 157 and 2-1 (201) were installed in September 2014; they are laid out in a 472 km geodesic line.

Correction to my previous comment:
The 85km section of border that runs along 65'30°N (NAD27) is not a geodesic line since the monuments all share the same latitude.

This changeset includes survey monuments between 120°40′51.00″W (NAD27) and 110°40′00.00″W (NAD27). The monuments were installed in 2011 and lie in two geodesic lines.
Just to be clear, I have been snapping the boundary ways to the monument nodes.
Previously the old boundary shown on the map had a maximum position error of around 10km due it not being drawn with geodesic lines.

I guess I can't edit my previous comment. The sharp turn is at Monument 4-30 (not 4-20).
Coordinates are 68°00′00.00″N 120°40′51.00″W (NAD27)

Nunavut Act stated that this part of the border should be at 120°40′51″W longitude.
One monument (4-30) was installed in 2011 at a 120°40′51″W NAD27. The SGB decided in 2014 or 2015 that the remaining monuments should be installed at 120°40′42.00″W (NAD27) to align with the centre tread of the Outwash River. The result is that the border now takes a very sharp turn at 4-20.

There is a small problem which I cannot resolve. The road signs appear to show a boundary formed from two straight lines.
One of the lines appears to be an extension of the boundary between Petit-Mécatina and Labrador; i.e. it is a north-south line at 57°06'32"W. The problem is that the actual time zone sign lies about 1 mile east of that line.

http://www.erichall.eu/images/1709/1709309.jpg
https://www.thetelegram.com/news/local/labrador-time-legal-or-not-154860/

FYI. The old shape of the time zone boundary in SE Labrador was clearly based off a KML file that I created and uploaded to the Internet in 2007. That was prior to the completion of the HWY system. Things have changed since then; they highways were completed and the government erected a time zone change sign (complete with poorly drawn map) along HWY 510.