I suspect the glitch has been in the game ever since the Aquatic Update (1.13). I have tested this in 1.16 and the current version and hasn't been fixed.

To trigger the glitch, dying in a one block gap from swimming animation (indicating drowning death) while in water (you must have a block above you to trigger it) tricks the game into thinking you are in swimming animation but your hit is reverts to walking hitbox. You do take suffocation damage while phasing through but you still can go through blocks, so long that there is a one block gap to go through.

(possibility of video being added soon)