In practice on the fediverse today, as @raphael says, most fediverse servers do custodial keys. Client-managed keys and signing are still possible though! See @silverpill's FEP-ae97 on client signing. LD Signatures are also relevant; they're not ubiquitous like HTTP Sigs, but still somewhat common, eg Mastodon does them. Also see the SWICG HTTP Sigs report.
cc @Julien51