On one of my machines I use the ArchLinuxArm packaged version of Anki which happens to be 2.1.15. I noticed that the changes page reads
After using the latest version, if you wish to open your collection with an earlier Anki release, please go to the File>Switch Profile menu item, and click on “Downgrade & Quit”.
The latest version is 2.1.26, but the downgrade function seems to be months old. What is the latest version of Anki I can use without downgrading (assuming one of my clients is stuck on 2.1.15)
the latest that just works with 2.1.15 is 2.1.22 (or 2.1.23 for macos which I can’t test).
In case you weren’t aware, this only matters if you are manually copying Anki’s data files around - newer clients can still sync with older clients via AnkiWeb, and export+import will still work.