Is demon summoning illegal

Not illegal, just inadvisable.

Demon summoning almost always goes badly for the summoner. And as an ethical practitioner knows and understands that what they do not only reflects on themselves (check out the rule of three) but it also reflects upon everyone who ever has or ever will practice magic. The world tends to believe that what the worst member of a group does is what all membrs of that group will do.

Legality is the least of your concerns if you intend to try summoning a demon. Also, the government does not believe in such things, and even if they do, they do not want to appear 'silly' by making laws to do with such things as demons, rituals, magik, etc.