The web updater indeed deleted the accessibility app on my instance when upgrading, as designed. For the record: Migration path was 24.0.4→24.0.5RC1→25beta4
Yet, unfortunately I hit an issue trying to use the web updater: After maintenance mode got disabled by the web installer (and prior to migrations), the browser kept blank yielding error 500 in dev console. nextcloud log says:
{"reqId":"<ID>","level":3,"time":"2022-09-03T23:20:45+02:00","remoteAddr":"<IP>","user":"--","app":"PHP","method":"GET","url":"/","message":"Declaration of OCA\\Talk\\Share\\RoomShareProvider::getSharesInFolder($userId, OCP\\Files\\Folder $node, $reshares): array must be compatible with OCP\\Share\\IShareProvider::getSharesInFolder($userId, OCP\\Files\\Folder $node, $reshares, $shallow = true) at /<path>/apps/spreed/lib/Share/RoomShareProvider.php#520","userAgent":"Mozilla/5.0 (X11; Linux x86_64; rv:104.0) Gecko/20100101 Firefox/104.0","version":"24.0.5.0","data":{"app":"PHP"}}
So i disabled spreed from the console:
occ app:disable spreed
After that, reloading the web updater continued and showed the migration todo list:
However, although I have a big screen: Without reducing zoom so I could not see or access that start button – the scrollbar is missing on that page … also applies to the (same, just modified) results page (I needed to zoom to 40% to see all, yet then I couldn’t read it any more 
):
When checking for Talk in /settings/apps I noticed that Talk/spreed needs to be updated to 15beta, and after updating, reloading (to get rid of “untested app” and activating it, all is good as new. I assume that after Talk 15 is released for NC24+25, it would have been updated prior to attempting to migrate, so this should go smooth then. Right?