one minor correction: the video says you can't tell what opcodes are used in unspent scripts.

this is true. you *can* however tell what version of scripts bitcoin is locked up to.

so theoretically you could remove support for a version of script once all bitcoin locked up to that script version had been spent.

there are currently 3 versions of scripts in bitcoin; none of them have ever been deprecated and all of them currently have funds locked up to them.