This is (still) technically possible but not recommended by Microsoft, or at least not officially allowed. They explicit say that you don’t enable the account and don’t give it a known password. Their recommendation is to give another user rights on the shared mailbox, like I mentioned above. You will have a problem with this configuration once they change the way it works and they can change it whenever they want as it was never supposed to work that way.
Exchange Online limits - Service Descriptions | Microsoft Learn
→
10 By default, shared mailboxes have an associated active user account with a system-generated (unknown) password. To block sign-in for the associated shared mailbox account, see Block sign-in for the shared mailbox account.
Every shared mailbox has a corresponding user account. Notice how you weren’t asked to provide a password when you created the shared mailbox? The account has a password, but it’s system-generated (unknown). You aren’t supposed to use the account to log in to the shared mailbox.
But what if an admin simply resets the password of the shared mailbox user account? Or what if an attacker gains access to the shared mailbox account credentials? This would allow the user account to log in to the shared mailbox and send email. To prevent this, you need to block sign-in for the account that’s associated with the shared mailbox.