Bluesky is neither decentralised nor federated.

I wouldn't let that stop you from signing up for bluesky -- in my experience it is definitely the best of the options for a twitter alternative -- but you should know exactly what it is you're getting into before you get into it.

And if you want to know exactly what you're getting into with bluesky, you should read this, which tells you more than you really want to know: https://dustycloud.org/blog/how-decentralized-is-bluesky/