Turn off permanently in Mail Organize by Conversation

Yes the system should generate a new one, but... you'll be required to setup all your Mail accounts again, quite a pain, but it is what it is.

You may want to rename it as a test first...

~ tilde indicates  your user library folder

Apple hid the Users' Library folders...

Method 1:

1. From the Finder, select the Go menu at top of the screen, and choose Go to Folder.
2. In the window that opens, enter ~/Library, and click Go.

There is a chance if you can Delete it without quitting Mailthat then oince you quit Mail it could possibly save an uncorrupted version of that plist.

I'm not on Big Sur at the moment, but I think it's likely this file is corrupt if Mail>View>Organize by Conversation doesn't stay uncheckd...

~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist

Your eMails should be safe if IMAP accounts, but you lways should have a backup or clone of what you have.

OK, good luck my friend.