You mean since part-FCL came into force?
Well, I have not been flying with the Wright Brothers 
but in the UK the PPL theory never expired once you “locked it in” by passing the skills test. I know for a fact this goes back to the 1980s at least in that you could stop flying for 30 years and not have to re-sit the exams.
The UK CAA has adopted a derogation such that…
The exemptions have now been incorporated at FCL.625(e) and FCL.740(b)(2). See consolidated Aircrew Regulation dated 8 Apr 2020 (link).
but in the UK the PPL theory never expired once you “locked it in” by passing the skills test.
Those exam passes expire after 24 months unless an acceptable application is made for the initial grant of the LAPL/PPL.
