The very first Roon Ready device still work and they are very old in the tooth.
RAAT SDK as @Suedkiez mentions is lite on features as it’s on the endpoint and the manufacturers are under no obligation to update the RAAT SDK on them once it’s certified as it’s not needed for it to continue to work nor is the operating system dependant on being updated.
Only the software side of Roon remotes and server has hardware that might become outdated as features are added that require a minimum version of OS or security updates require it. Major changes to RAAT are done on the server side and don’t affect the endpoints which is how’s it’s been designed. If you buy a Roon Ready device today it would continue to work on future versions of Roon.
In the case of DAPs the only thing that might not work over time is Roon Remote or ARC as they will have certain requirements. None of which will affect Roon Ready. I use my phone to control my FiiO Roon Ready DAP rather than using Remote on it as it’s a smoother experience and I can leave the DAP alone once playing.