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

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If you have a fediverse account, you can quote this note from your own instance. Search https://mitra.social/objects/019783a3-0e97-9eae-8dc9-f22bf669c751 on your instance and quote it. (Note that quoting is not supported in Mastodon.)