Note: I never pressed left there, I just performed a Backwards jump and then Mario did a bottom corner clip there. And catching the walljump pixel was intended there. Maybe this can be a "mid-level" strat for MW since Mario doesn't slow down by doing this. It seems that Big Mario's upper hitbox part is buggy with collision detection, since when I do a backwards jump with only Mario's head hitting the block, he starts being constantly pushed in and out of the block. Would be cool to find a way to make this useful in a run excluding MW. Maybe warpless?

This is easy way I was taught to get to minus world. Yes it has been found before. I imagine if you look at the minus world leaderboard, the majority of submissions, certainly in the slower half, use this method

This method has been known about for ages. Here is a video from 2010: