Perhaps a more interesting question, and one which I cannot quickly answer (nor am I going to go pinging huge swaths of the public internet), is are there any hosts in any version of HOSTS.TXT that are still on the public internet in the same location?

Hard to answer the question.  Same physical location?  Same hardware?  mit-prep.arpa is prep.ai.mit.edu which is a CNAME to ftp.gnu.org  Theseus's ship?