There are only a small handful of tetris players even capable of maxing out the score. Maxing out the score also doesn't lend itself to a speedrunning nature. It takes proper building to get a maxout, and a very different play style than one would opt for in a speedrun. So it's better to have a lower score or line goal for the sake of accessibility in the end.

Could be a neat idea for a category extension leaderboard though, not that this game particularly needs it.

While no one is currently timing maxouts with LiveSplit, maxouts can be judged by using number of lines to get the maxout as a pseudo in-game timer. Generally speaking, the fewer the lines needed to maxout, the better the maxout.

This makes for a natural leaderboard based on fewest lines needed to maxout. I don't see a reason this couldn't be tracked here on SRC. This is what TetrisConcept uses for ranking on their leaderboard.

Since SRC is generally more well-known than TetrisConcept, this would be a great centralized location for tracking maxouts with a very simple way to rank them by efficiency.

In terms of "only a small handful" of players able to maxout, there are currently over 50. This seems to me to be a more than sufficient number to get a category going.