ISIS is a sanctioned entity. Knowingly allowing them, or anyone identified as a member, to post on Twitter, would likely be illegal under sanctions laws.
To the extent that Twitter’s approach to ISIS is not mandated by sanctions laws-I don’t see Musk changing Twitter’s corporate policies on ISIS-he has zero sympathy for them and no doubt views them as a threat to humanity’s future.
Musk is likely good news for Donald Trump, Babylon Bee, Jordan Peterson, Libs of TikTok, etc - but no change for ISIS.
I still wonder about the legalities of Twitter allowing them to use the site, given they are still under US sanctions. It is possible, however, that the US government has (quietly) asked Twitter to allow it, as a diplomatic/political calculation. Sanctions concerns disappear if the government is asking you to disregard them (they can give you a formal legal exemption from them - even secretly; even without a formal exemption, if the government asks you to do something, that is an estoppel against them taking legal action for acceding to your request.)
My neighbor Sam down the street likes to say the n-word sometimes and thinks offering hormones to kids is grooming. Still, I don’t think all the Sams in the US would ever be able to do as much material harm by having access to Twitter as ISIS does in one afternoon. Could be wrong.