Alternative clients do exist but OWS has a history of taking them down and/or making protocol changes to block them, similar to AIM back in the original days of Pidgin.

Those are unsanctioned

Such an inflammable statement, let's sanction those then. :)

Unfortunately, whether or not a client is sanctioned for use on Signal-owned infrastructure is entirely up to the Signal organization, and they have been rather hostile to such clients.

Sorry I was just making a joke about how sanction has a double meaning. :-)