This is pretty much a complete guess, but it might have been done with something like dblue glitch? Perhaps the gater effect? It would make sense since a lot of the other elements of that chorus seem to have dblue glitch effects too (like tapestop and perhaps shuffler too). If you want a similar-ish effect, definitely try it out

Sounds like a lowpass filter using an LFO with speed decreasing over time to control the cutoff. Also sounds like the LFO's waveform morphs from Sawtooth to Triangle as it slows down.