Only Mastodon and Iceshrimp.NET didn't accept actor documents without @context. It is not clear why Mastodon requires it, because Mastodon doesn't use it, as far as I know. The related issue has been open since 2018.
Meanwhile, @context has been restored for @nomad and now it can be followed from Mastodon too.
RE: https://mitra.social/objects/0197759b-8413-2f5c-d7ec-83509cedf468