I’ve added a new page to the switch2osm guide: “Manually building a tile server (Ubuntu 21.04)”. The work that the Debian developers have done to include the required software inside Debian 11 is now in Ubuntu 21.04, so there’s actually less setup work needed than in previous Ubuntu versions.
Ubuntu 21.04 isn’t an LTS release, so Debian 11 or Ubuntu 20.04 are still the recommended options for servers that are expected to stay around for a while. One of the regular faux pas with the Natural Earth download site has just been and gone, but the guide includes a workaround in case problems reoccur there.
Unrelated to Ubuntu 21.04 directly, there’s currently a bit of an issue with the lack of feedback that part of the OpenStreetMap Carto setup gives to the user, but work is under way to improve that (I hope things will be able to return to where they were last year with this). I’ve tried to explain here and in the other guides that it’s to be expected that “things just go quiet for up to an hour”, and that eventually it will complete.