I attempted to download the fix_upgrade branch from the link, but it does not appear to be there any more.

Yes, that's true ...

[EDIT: I see there are updated scripts now in the master branch, and suspect you've merged all fix_upgrade files into master. Would like to confirm I should use master now instead of fix_upgrade.]

... because I meanwhile merged the changes into the master branch. So, if you are willing to support the project by testing the changes, you can carry out the test with the master branch.

I tested myself an upgrade starting with version 2.4.20, 2.4.99.1, 2.4.99.3, 20220508.1 and 20220529.1. I installed a copy of my testing forum on version 2.4.20 in each case, upgraded it to one of the interim versions, that should be the starting point of the test and did the upgrade to the newest target version. In the end I found no issues with the changes in the database structure (after several corrections).

Nevertheless, I recommend to undertake the upgrade test with a copy of your live forum (script code and database). I fyou find issues please report them here or, if you have a Github account, as an issue.

Thank you for your effort.