ie it will forever be compatible with whatever official client is in use in 15 or 20 years time? a tx created with locktime today will always work in the future?
since bitcoin is not centralized, there is no centralized entity to release "official" anything. all we have is consensus that the entire network has to agree no matter what client they run. and since things like LockTime are enforced by the protocol to change them we have to create a fork and that is not going to happen and even if it does you will see it if you follow bitcoin (as in checking every couple of months).