Going by memory: a hook was left in 2.1 for backward compat, because SCEditor should have its own way to add buttons. TBH I don't know if the hook is still around... lol
The whole thing though looks a bit pointless after these changes, because in fact IIRC the only thing ```$context['bbc_tags']``` does is define the buttons to show and the order... **but** if that is all about that, probably do it in javascript is the way to go.
Unless I'm too tired to think clearly.

/me goes bed now