Yeah, it's one of the oldest known tricks since it happens on accident occasionally. However, there hasn't been a use for it in speedruns so far.

i think there was a use in 12 where you jump earlier and corner clip on the platform with the end goal

Wouldn't you get there at the same time horizontally?

Yeah, the clip in level 12 is possible, but it doesn't save time because you get to the door at the same time either way. Currently, the only level with a strat where a corner clip saves time is level 19.

Level 18 now has 2 possible time-saving clips!

What was the now deleted user talking about?

@5ber From the comments in the original thread, I'd say he was talking about the wall clip, a trick that allows you to place yourself inside the wall of any level, but it requires specific timing.

Here is a video where they explain it: