TL;DR: head over to http://taghistory.raifer.tech/ for usage graphs of arbitrary OSM tags over time (by number of OSM objects).
OSM doesn't have a fixed set of object categories. Over time, a more and more faceted and diverse set of features got mapped in OSM, thus the amount of different tags grew. At the same time, sometimes, tagging of a specific thing changes: Features that used to be mapped with one tag, get newer, better and more refined tags. That's OpenStreetMap evolving.
Of course, OpenStreetMap is also still growing, but not all the tags are getting more widely used at the same pace: For example, while it's quite possible that most of the world's railway stations are already mapped in OSM, there are still many juicy pastures left to be mapped out there.
While there exist superb tools to get to know about the current state of all tags used in OSM (Taginfo most notably, but also the Overpass API to some extend), until now it was quite difficult to get oneself a good picture of the data evolution process. For example, questions like: from when on a specific tag was getting used, when an obsoleted tag got taken over by a different one or which tags got more traction lately are difficult questions to answer with OSM's current tool set.
For some of these questions, people programmed their own solutions, each answering their own question, like how many km's of Italy's roads were there in OSM over time (link), or how many buildings have been mapped in Austria (link). Similarly, the OSM-Analytics platform has recently started to provide such statistics for arbitrary regions for a limited set of map features (currently one can choose between buildings and roads, but there are plans to add more in the near future). What all of those tools have in common is that they can't handle the full variety of tags that's so essential in OSM.
To step into the gap between tools like taginfo (where the full variety of OSM's tags is so beautifully visible – stay tuned for Jochen's talk on SOTM in a couple of weeks!) and the more specialized tools like osm-analytics, I've created taghistory which allows one to get a historical usage graph for each of OSM's tags (with daily granularity) and to compare different tags against each other:
The tool is currently in it's very early stage, the're many things to do and improvements to be done. It's also important to note that the historical usage of a tag is currently only defined as a the respective number (count) of OSM objects! That's similarly to the statistics produced by taginfo, this metric is subject to the some limitations, most notably the effect that one cannot directly compare the number of tags used for different linear and polygonal features such as roads, land cover, etc. because such features are typically divided up into many OSM objects of different sizes. For example, an existing road may be divided up into two pieces when a new turn restrictions is added, resulting in that the count of each of the tags used on the road (even obsolete ones) is increased by one in the OSM database. That means that one needs to pay close attention when comparing tags that are typically used on such features, even when comparing subtags that are typically used on the same kind of parent object (e.g. different values of the highway tag).
That being said, have lot's of fun while digging into the depths of OSM tags' history. Here's the link of the tool again: http://taghistory.raifer.tech/ (and the link to the project's source code repository and issue tracker: https://github.com/tyrasd/taghistory). What's your favourite tag? I find the
created_by graph quite interesting: