When I have heard it ["deprecate"] used it is related to provisions in a standard that are allowed for existing work
I have no idea what you're trying to say here.
Unfortunately, not all of us have degrees in music and judging by comments in Norman Del Mar's Anatomy of the Orchestra, even professional musicians do not like the practice
Professional--and amateur cellists--are not going to see it in their parts. It is only encountered in certain old scores.
Whether it counts as 2 separate clefs or one is something that we can argue about all day
No, it's not. The treble clef is one clef. That isn't subject to argument.
but to the man on the Clapham Omnibus, i.e. the original poster then they may as well be 2 different clefs because people who are at the level of the OP do have problems and need to be warned. It would be irresponsible not to say anything based on a point of pedantry.
All the original poster needs to know for now is that the cello mainly uses the bass clef. You are the one trying to confuse him, and you're obviously doing it to try to show off. I did not withhold any information or advocate withholding any information, however; I merely corrected your errors.