Unilateral changing of contract terms is indeed naughty. However, how many passengers would really challenge Ryanair cabin crew by insisting on their rights, especially when calling the police is threatened for not wearing a mask ? Flying with FR usually involves multiple jurisdictions - it's a bit of an unknown to figure out which country's laws do or don't apply. I very much doubt that the average police officer in Europe is remotely interested in the legal niceties of what details were in the contract at the time of booking versus what is in the contract now.... most police officers at an airport will just arrest first when cabin crew claim a passenger has been disruptive, and ask questions a few hours later. Do you really want to be held under arrest for a couple of hours while your flight departs, only for the police to decide to let you go without charges ? Challenging FR through the police and courts to seek later recompense can be an awfully expensive and time consuming business
Thus, FR pretty much can insist on everyone wearing a mask whether the requirement is legally binding or not... and get away with it