You cannot directly see who removed you from admin in WhatsApp
WhatsApp does not send a notification showing which admin removed your admin rights
Check the group info to see whether you are still listed as an admin
Ask other group admins if they removed you
Review recent group activity or messages for clues, if available
If you are no longer an admin, only another admin could have removed your admin status
If there is only one other admin, that person is the one who removed you
If there are multiple admins, you cannot identify the exact one from WhatsApp alone