I actually don't agree with this idea. I think there are legitimate reasons to play other versions of the maps. Or even in cases that aren't legitimate I don't think it makes sense to prevent people from running them. Take koth_pro_viaduct_rc3 and koth_pro_viaduct_rc4, for example. Many people run rc4 now, but many still run rc3. I have them both downloaded. I still see people run old versions of snakewater as well because there are some things in them that are different from the final version.
Overall I don't think there's anything inherently wrong with running old versions of maps. You can always point out to the lobby leader that its an old version, but as I said, they may very well know that. If someone wants to use their own server and their own server time to run the old map, and people are joining the lobby for it and willing to play it I don't think there's anything wrong with that.
As a side note, however, as I know there are a lot of less experienced players who may just be starting out, or may have little interest in the maps being played, I think it would be beneficial if there were a marker for more common maps with different versions stating if it is the most recent version or not right on the lobby. I'm not sure if this is desirable, as it would need to be updated a lot as new versions of maps come out, and there will always be obscure maps and the possibility of omitting maps. I don't think, though, that it would be very much more difficult than displaying an image of the map for common maps as is already done.