While it might seem easy on simplest cases (e.g. a road connects to the building), it becomes problematic on things which are not so clear about direction. If the door is connecting two buildings, what would be entrance and what would be exit building (both buildings might have other doors leading out on different streets)?
Or if outside between two pieces of land (or underground, connecting two tunnels) there is turnstile going in one direction only, what is entrance and what is exit? What if one leads to plane and another to train, what is entrance and what is exit?
I’d rather prefer adding a simple way with oneway:foot=yes leading to that entrance as unambiguous and clear solution for marking that minority of one-way-only doors/turnstiles/whatever.
Well, if I had time machine, I’d like to deprecate more than 50% of the entrance=* values, not just exit. Nowdays however, I have at least 10066+ reasons (and growing daily) not to deprecate entrance=exit.
I think my default reply to sentences containing word “deprecate” in OSM context should be “And how many months of your life are you willing to dedicate solely to accomplishing that suggestion in a reasonable way?” Just to put expectations into realistic frame… 