No, it cannot even be considered a free speech issue (except insofar as Trump proposes to censor Twitter). Those of us in the con law/democratic theory community, and everyone else in the universe who is even semi-rational, use them term "counterspeech" to describe what Twitter did.
Traditionally, counterspeech is seen as the virtuous alternative to censorship---as the thing that us snotty free speech people tell those who call for their opponents to be censored to do instead. John Stuart Mill would jump up and down and pop champagne in celebration of what Twitter did.