In case you're tired of trying to come up with an answer, you can find the correct one here.
The hardest chess problem in the world?

Promote to a golden magic king. Then can walk into check because all moves are legal for one including teleportation

maybe some kind of castling? Castling´s rules were pretty different until the creation of the FIDE rules. For example, before FIDE´s rules even vertical castling was theocally posible . Wont surprise me hear about diagonal castling... And in this case, it is mate if a diagonal castling is applied. Who knows...

are y'all reading the hint? it PREDATES modern chess rules.
@CharlieYZ no wonder the analysis says it's fake! the analysis only analyzes modern chess rules.
@Samurai-X same for you. in this age, it's an illegal move, but it wouldn't have been a few hundred years ago. it's m1 but not with modern chess rules
are y'all reading the hint? it PREDATES modern chess rules.
I'm afraid it always was a joke. Nobody has shown any pre-modern publication with rules that allow it. Of course, before FIDE existed (pre-1931) there might be someone claiming to have visited a small town in Kazachstan where they played by different chess rules. That's the way frauds and fantasists weave their web of lies. It is just a myth created to have a myth
.It's worth mentioning that there is a small, modern stream of joke problems created by otherwise decent chess composers. Everyone likes a day off work and that's how you do it in chess. Commonly they don't tell you which joke rule(s) they use but they let you know it is a joke through text or context ! Or, you create a new fairy type (chess variant) by publishing the changed rules. There are thousands of fairy variants used to expand the domain of chess compositions.
Over the weekend, I came across this puzzle, which I found too difficult to solve. It's a mate in one - yes, you read that right. Good luck!
Hint: This puzzle is so old, it pre-dates the publication of FIDE's official rules of the game.