What are line-of-sight limitations of UHF RFID readers?
If you could place a wire loop antenna around the door you could do this with low frequency RFID.  Three turns of 10 AWG wire surrounding a doorway makes a nice passageway detector.  TIRIS readers were recently discontinued by Texas Instruments but ProtagD in Germany makes clones of them.

Shadowing will be a challenge regardless of technology.  You want a large read zone to have good detection efficiency while a smaller read zone would limit shadowing.

Low frequency RFID has a better defined read zone while UHF has longer read range which could increase shadowing.  UHF also will sometimes pick up a rogue tag that is quite far away (in another room).

Low frequency RFID without anti-collision will only detect the strongest tag and block others.  UHF has anti-collision which means it will detect all tags in the vicinity, again increasing the opportunity for shadowing.

Some readers can measure the tag signal strength but this is a poor indication of distance since the strength changes as the tag changes orientation relative to the antenna.  A tag further away from the antenna could have a stronger signal than one closer that is not oriented as well.

The primary advantage of UHF in this application would be that the antenna is a small box you can mount above the door as opposed to LF which would need a wire loop around the door.  By pointing the UHF antenna down it might work to limit the zone to just the doorway area.

HF RFID probably would not be a good fit for this due to short read range and limited antenna sizes for scanning large areas.