Checkout of source code to previous stable?

To check compatibility I cloned NC (server), as in Development environment — Nextcloud latest Developer Manual latest documentation.
When I try to git checkout to previous stable like stable30 I get:

An unhandled exception has been thrown: OCP\HintException: [0]: Downgrading is not supported and is likely to cause unpredictable issues (from 32.0.0.0 to 30.0.6.2)

I am aware that NC when installed does not allow downgrade. And really in config.php it is still 32.
But is it not the point of source to be able to do it? How?

Hello.

Generally speaking, git should be able to check out any commit. The message you posted comes from the NC server. You have installed the master branch (actually installed not just checked out) and then checked out any previous version and opened the browser (or ran an occ command).

Is this a pure development instance and you can remove all data inside? You might delete the data folder plus eventually reset the SQL database if you use anything but SQLite.

Chris