Does the version difference (21 → 29) matter when restoring the wallet?
Not at the moment.
It'll just warn you that it'll soon be deprecated in the future versions.
For v29.x, it'll still accept your legacy wallet.dat file.
How can I use/rename the wallet.datxxx.bak file so that Bitcoin Core recognizes it again?
The new client asks for a wallet name. How can I find out what the original wallet name was back in 2021?
You're already on the "
name" part, so I guess you've used Restore and the wallet backup is accepted.
As for the name, it's not asking for the original wallet name but your preferred name of the wallet once restored.
And your backup wont be modified by Restore, it will simply copy it to your "
wallets" directory.
Now, reply if you encounter an error after that because it means that there's something wrong with it when Bitcoin Core tried to load the restored wallet.