Yeah it would probably be illegal to use the data for anything in the event of a GDPR removal.
They do technically still have it in their backups, most likely. It should be covered in Reddit's ToS.
According to France’s GDPR supervisory authority, CNIL, organisations don’t have to delete backups when complying with the right to erasure.
Nonetheless, they must clearly explain to the data subject that backups will be kept for a specified length of time (outlined in your retention policy).
If you decide to go down this route, you should bear in mind that other supervisory authorities might be stricter and that you must be able to demonstrate that it’s impractical to delete backup data.
https://www.itgovernance.eu/blog/en/the-gdpr-how-the-right-to-be-forgotten-affects-backups-2
I switched to Obsidian not too long ago.
For my needs, Joplin was a good open source alternative.
Between the two I went with Obsidian because, while the apps are closed-source, the data is accessible. All your notes are just stored in plaintext (with markdown) as simple files in a directory structure.
Joplin, in contrast, uses a SQLite database which adds a layer of complexity.