The Tetris board is 10 blocks wide, so to be able to play the full field, you need to be able to tap left or right at least 5 times before the block lands. Without tapping at super-human speeds, the fastest way to move the blocks that fast is by holding the left or right button down and letting key repeat take over. When that’s someone’s approach, level 29 becomes fast enough that the pieces can’t reach the edges of the board, so it was traditionally called the “kill screen”, since it wasn’t practical to make progress from there. While it’s possible to get to 999999 before reaching level 29 with that play style, it just didn’t happen all that often, and only by the most elite players.

The last few years have seen people learn how to “roll”, effectively lightly pressing left or right and then drumming their fingers on the back of the controller, getting discrete taps far faster than typical. That lets modern players continue playing at level 29 speeds, and since the speed never increases again, they can play until the game gets to situations that could reasonably be ignored as impossible in the 80s.