"Indefinitely suspended" implies that Twitter might, on its own, lift the suspension at some point in the future.

"Permanently suspended" (with chance to appeal the suspension in the message) means exactly what happened here.

"Indefinitely" is reasonable, but doesn't make it clear that they have finished their current investigation. So it's also imperfect.

Just "suspended" is not a good option at all, because it says nothing about duration.

"Indefinitely suspended" might be better, but just "suspended" is probably going to be very confusing. In education, for example, "suspension" is a temporary ban, and "expulsion" is a permanent ban.