I only go back to 2002, the first year for SM in what is now the GreatLakes Division (originally CENDIV Area4), so my experience is only in that division and for the first year the spec tire was open (I ran the Kumho VictoRacer, which was an OK tire), then it was the RA1 for a year or two, then it was open again (for regional competition). I have no idea what the regional/divisional rule is now for Greatlakes as I moved south several years ago but I think it's the spec Hoosier.
I'll admit my memory of specifics 18 years ago can be a little foggy.
central division SCCA has an SMT class, spec miata open tire class. you can run anything. most people use it for cheaply double dipping and just run hoosiers. ive seen toyos run as well.