In ICSCC Pro 3 automatic transmissions are specifically allowed. So the ICSCC conference rules have no prohibition on automatics. Then you have to look at individual class rules.
ICSCC ST class is totally based on HP/Weight ratio. The only mention of transmissions is that an H pattern syncromesh tranny gets a bonus weight reduction factor. Under ST rules it seems that an automatic would be legal, but an 818 is not because the class does not allow tube frame cars.
ICSCC SPU-M-O classes have no rule against an automatic. SPM is where I will race an 818 when I build it.

But this is all just theory in an 818 because I doubt it's possible to convert any Subaru automatic transmission to front wheel drive only (rear wheel in an 818). If it is possible it would make it a lot easier to tune a 3.0 - 3.6 since the Subaru USDM ECU for those engines is only for an automatic tranny car. You could tune them with RomRaider. You wouldn't need an aftermarket ECU unless you also turbocharged it.