I'm reading the Tetris guidelines from here: https://www.dropbox.com/s/g55gwls0h2muqzn (linked by the Tetris wiki). On page 24, it says that in the classic lock down ruleset, the lock down timer resets only when the y position of the piece decreases. Does it mean "goes lower than it had ever been" (like in the last 2 rulesets) or "lower than its current position"?
I'd assume "lower than its current position", as most guideline games allow you to keep rotating, or even just shifting left/right, to reset lock delay an infinite number of times. Some games limit how many times you can do that though, like World rule in TGM3, but the same rule applies, just a limited number of times. "Goes lower than it had ever been" is "step reset", as first seen in Sega's System 16 Tetris, the ARS games by Arika (with slight exceptions in TGM3 Classic; see the wiki for details), and perhaps others that were inspired by Sega's Tetris, that use Sega rotation for basic rotations.
@nightmareci: this is specifically referring to Classic Lock Down (have you read the document before?), not Extended or Infinite. I don't think any game has implemented infinite as the default in around a decade. As for the answer to the original question, I know that there are some games in which Classic is available as an option such as Ultimate and New Century (not sure what else off the top of my head, can someone help?), and I'm fairly sure that for consistency with the other rulesets it should be "goes lower than it had ever been", but I don't have any way of testing this at the moment. This should be fairly easy to check using a suitable setup if SRS is still enabled (if kicks are disabled this is not an issue, of course).
Yeah, classic lockdown is definitely intended to be step reset. The specification is a little unclear. Tetris New century does let you enable classic lockdown. It's easily tested with an s or z piece, which will be kicked up and fall down repeatedly if rotated 360. without requiring forward progress beyond where it's been before, you would still have infinity, which classic is intended to prevent. I'm 99% sure that lowest y coordinate so far changes is the trigger for resets in classic lockdown, if you wish to program a Guideline compliant Tetris game with Classic Lockdown. Oddly enough, Classic Lockdown is not the original. the original, used by NES and most prior games i know of, is gravtity lock, which makes the piece lock when gravity attempts to move the piece and is unable to. This allows slides and twists, with them being easier at lower gravity. Some other game use contact lock, which means pieces lock immediately upon touching a surface. This is very bad, as it prevents slides and twists. And one more scheme is entry reset, where lock delay goes down while in contact with a surface, and is only reset when a new piece enters the field. There are a few games that do this, but i'm not sure what they are. They usually have a longer lock delay to make up for the lack of resets.