In older releases of macOS one could create .metadata_never_index in any folder to prevent it and its children from being indexed. This seems to have stopped working at some point. Does this or another similar method work in Catalina and Mojave?
Note: I am not interested in adding .noindex to file/folder names.
Your suspicion is correct. Apple has (criminally) went ahead and removed support for not indexing an entire folder. While I pray they add this back, its unlikely, as they have made the same change in the file indexer for iCloud Documents and Data Sync (.nosync files in folders no longer work, you need .nosync in the file name).
You said you don't want to change file names, so unfortunately there is nothing you can do.
If you could settle for the illusion of unchanged file names, you could rename the files to be hidden (start with a .), and end with .noindex, and then create a symlink to this hidden file from a link with the originally desired file name.
This answer regarding the equivalent issue for iCloud Sync is along the lines of what you'd need to do (note the accepted answer on the linked question is outdated and no longer correct)
Please see my recent post about problems with folders (or volumes) with file ".metadata_never_index_unless_rootfs"). In Catalina 10.15.7 it does work, and caused me a lot of problems after a failed update (to Monterey) left such files at the top of most of my volumes.
