There's an extremist viewpoint on free speech that it is a categorical good, divorced from any societal utility or harm, which elevates it almost to a point of religion.

It's always interesting to me when I observe it in action, because not even the US legal system---a system that, among the systems of the world, enshrines free speech as more untouchable than most nations---agrees with this absolutist premise.

You mean the first amendment being trampled on by a president threatening legal action against a private company?

The first amendment isn't a catch-all freedom-of-consequences thing; if (to use a straw man argument) Twitter did not remove ISIS propaganda, the US government would shut down.

While technically proclaiming the virtues of joining an army to fight for them can be considered freedom of speech and should be protected, in practice it's not because they're a deplorable terrorist organization.

This argument makes me pretty uneasy, since it seems like it can essentially be used to censor whatever you want. If e.g. the people fighting for climate justice get branded as ecoterrorists, wouldn't removing their 'propaganda' be ok under that line of thought?

I think the right to free speech isn't some enshrinement of the right to spew garbage, but the realization that restrictions of free speech can very easily be turned against 'good' causes.