I think it's highly dependent on the user. A technically inclined HN user probably knows people who shifted to a tech Mastodon instance (like infosec.exchange).
As a personal anecdote, the people I know who cut back on Twitter did so long before Musk due to general social media burnout.
Quite a lot of my feed was Popehat, Tqbf, and Mjg59, who have all left the platform (more or less). Also a handful of low volume, personal friends went to Mastodon or retired from the platform.
As a personal anecdote, the people I know who cut back on Twitter did so long before Musk due to general social media burnout.