In bluesky I think those effects mostly lie on the side of which client people use.

The protocol is extensible and you can add new post types and formatting options by creating a new schema/lexicon, but these would only be readable by other clients which supports it. I hope they’ll be able to add some general “category template” lexicons so a graceful degradation scheme can be implemented to support compatibility without hindering 3rd party development.

To protect against a PDS server going bad the client could assist with automated account migration (the new PDS doesn’t need to understand the lexicon of your posts to be able to migrate them intact), even if the old PDS won’t cooperate (the client could maintain backups for you to make migration quick). But if you don’t control your keys separately then a bad client update could make your account unrecoverable, similarly to a mastodon server going bad.