I have a 2015 vaccination which has been endorsed with a "Valid For Lifetime" stamp in 2020. It has been accepted in both Kenya and Tanzania without any comment.
My wife has a 2010 vaccination which has been questioned in Tanzania, on the basis that only vaccines issued after 2016 have lifetime validity and not those before. However, if she got the same "Valid For Lifetime" endorsement I did, it would be fine.
TL;DR - they make it up as they go.