Can system reserved memory be recovered after debloating? Say I have a rooted phone, and I "debloat" it by removing some system apps. This means removing apk files from /system folder. On the other hand, user-installed apps on internal memory go to /data/app folder, according to this answer.

Does this mean the system reserved space is still not available for app installation unless I use something like an app-systemizer (that forcibly stores apk files in /system folder)?
You are totally right, removing pre-installed apps from the system partition has absolutely no effect on the free space of the user data partition where apps and their data are stored into.

The only chance to be able to use the free space of the system partition would be to decrease the size of the system partition and use the now unpartitioned size to increase the user data partition. However these complex operations may end up in a total data loss even if the used tools support to perform it without data loss.