Microsoft allows you to block O365 shared mailboxes, and recommends that you activate this option by default, specifying the following:

"Signing in: A shared mailbox is not intended for direct sign-in by its associated user account. You should always block sign-in for the shared mailbox account and keep it blocked". Microsoft article source

The shared mailbox is then displayed in Entra as "sign-in blocked".

If this option is enabled, the shared mailbox will not synchronize with the Letsignit platform, as it is considered inactive.

If the option is enabled, however, it is possible to synchronize them by selecting synchronization of inactive users in your synchronization edit, but beware, this will bring up all your inactive users.

This article (part 4) gives you more information on synchronization editing.