I have already posted this many years ago but i did get a letter from Disney about exceeding the 20 rentals. This happened when i was buying lots of contracts with loaded points that i didnt plan on using. I also thought it was 20 rentals in a year not 20 in a rolling 12 months. I called up Disney explained what was happening and everything was fine. So they were certainly checking back then, 2012?
Now it is certainly simple to never exceed that limit by having different memberships.
Its also relatively easy - if Disney wants to do the work - to link those memberships together by name and/or address and/or corporate entity and all sorts of other things. Fraud software does this constantly - want to send a TV to your college student, Best Buy makes sure the person you are sending it to at that address is linked to you somehow before they ship it - if not, then they text you with "did you really intend to send a TV to this address?" This sort of check could be done to make sure your points aren't being used by strangers, it could be used to link points on different contracts to the same individual.