The unique hash for the RSS is generated in this order:
If the user is a guest, it generates a random string.
If the user is logged in and has a password hash, it generates a unique key based on it. (People who login with a 3rd party account, like FB or X, might not have it.)
If the user is logged in but has no password hash, a random unique hash is generated for their account, and there is no way to edit it. (It would require a modification, or you can edit it directly in the database.)
So, to make a quick summary, as long as you change the account's password, the unique hash is regenerated and the old URL won't work anymore for them.