I can only concur to the posts here; I have been to a triple-digit number of tournaments as a player, organizer, umpire, technical assistant, and spectator, and have never once encountered such a rule. There is certainly no registration of handedness in any official BWF (or German) tournament.
So the interesting question is: How can such a rule exist, and can it be enforced in any way?
The tournament organizers or accredited press can ask you to fill out a questionnaire for **promotional** purposes. For instance, BWF mandates players to be available for interviews in §4.3 of
the Player's Code of Conduct and §1.4 of
the Player Commitment Regulations.
These promotional materials do not have any bearing on the game; if you say *weakness: serve* and win the match with 42 immaculate serves right on the line, you're of course not being punished. You can decline to answer or answer
"I do not want to give out that information lest it advantages my opponents".
However, §4.2 and §4.3.3 of
the BWF code of Ethics could be interpreted as requiring players to be somewhat serious in media communication.
But how about having an actual rule requiring to play with a preregistered hand?
Any official tournament where top-level players can participate must be sanctioned by BWF, who will require the use of official laws. Similarily, official tournaments of the federation (say, regional or district-level championships) have to be sanctioned by the national federation, which will require their translation of the BWF laws as well. For more information on sanctioning, refer to §§3-4 of the
General Competition Regulations.
However, anyone can of course organize a private tournament without any affiliation to any official badminton organization. There, the organizers can make up their own laws - if someone wants to play under blacklight, with heavy training rackets only, or play a round of chess after every game, nobody can hinder them to do so. However, international-level players are generally forbidden from participating in these tournaments. This must be the kind of tournament you were playing in.