For RAIM to work the receiver needs to be able to see a certain number of satellites (5,6?) so that it can calculate multiple positions, realise that there’s an error somewhere, work out which satellite is in error and discard its input.
If it’s known that satellite coverage in an area is going to be limited for a period of time so that RAIM won’t work then RAIM outage is advised.
I have no view on the SBAS thing. Intuitively it would seem logical that it would work because of the corrections put in by SBAS, but I really don’t know.
HtH