I just noticed something others might have seen a while ago - the new PRO website no longer lists historical statistics for active Referees and ARs. There used to be the date for their first game in MLS, # of games per year, and some career highlights - nothing there anymore. Turns out to be surprisingly hard (unless you have a database - though Transfermrkt might be good enough) to find out when someone started in MLS, for example.
Additionally, worldfootball.net, a German site which I had referenced for 15 years for ref stats, going back to the start of MLS (though not nearly as accurate as @ManiacalClown , e.g., giving Christina Unkel credit for a Ted Unkel game and vice-versa), removed all AR stats and history during the MLS off-season. Referee history remains.