Apparently the OSA still applies to some of the documents these people originally worked on (including the specifics of Bombe, etc) are still covered and rumor has it that GCHQ still holds copies of the original decrypts, containing information that may be relevant even now because it relates to people still living.
Last I checked, some of them *may* be released around 2024 but this is subject to official approval.
If the classified patents issue is anything to go by the "Fifty Year Rule" may be more like the "Whenever it becomes irrelevant due to technological advancement Rule" and some mathematical and fundamental physics discoveries may be affected by this. Never fear, most of them are related to obscure topics which probably have no relevance to everyday life.
I've run into the patents issue before, if you accidentally rediscover something that is classified you may or may not be asked (politely) to sign the OSA, hand over original documentation and sign an additional document stating under penalty of perjury that they are the only copies.
Just doing a search for certain terms of interest (tm) may get you visited by some folks or strange phone calls etc if reconstructed search history implies getting close to something you shouldn't.