Here's the response I received from ConnectWise:
"While Kaseya 9.3 is not listed on our ConnectWise Marketplace as a supported version yet, we don't expect there to be any compatibility issues. If you do see any issues, please let us know and our team will be able to review further."
I believe the ConnectWise MSP Kaseya Web Service Installer doesn't actually use Kaseya's APIs - it implements it's own set of functions that directly query the ksubscribers database (Boo!) using an account with sa privileges (double Boo!!) so unless the database schema has been significantly changed their functions should continue to work.
If you have a 9.3 beta/test server perhaps try installing the ConnectWise web service installer and then bring up the test page as described in the documentation linked above - if the functions there returns results correctly I'd expect it should all work fine once put into production.