This AlAppExtensions issue concerned an event parameter that was added hastily without thinking and is not actually useful for anything:
https://github.com/microsoft/ALAppExtensions/issues/11996

Stan P explains there how parameters can't just be removed because users might be taking them, even if they're not using them, or if they THINK they are using them but not really getting the expected behaviour.

He explains how the current solution would be to add a 2nd publisher, obsolete the 1st one, and eventually remove the 1st one - leaving messy code in the interim and meaning the 2nd might have a non-preferred name.

Could there be a way to deprecate individual arguments, at least for events? That way we could signal clearly that they're not used/useful, omit them by default when looking-up events, warn if still present in old code or manually added, etc.

This would allow deprecating/obsoleting only individual parameters/arguments without having to obliterate the entire publisher. The 'removed' parameters could be xcluded from auto-populating upon event lookup, warned against if added manually or via Intellisense, etc.