i presume it wasn't in the design spec as a) it's an interaction behaviour, not a visual design behaviour, and b) it was also a thing in holo design (& older[?]), so they didn't consider it part of the "material design spec"?
Yesn't. Material Design 1 and 2 guidelines have a bunch of sections regarding interaction, way more than M3 (although M3 guidelines aren't "finished" yet), but they lack a section regarding that gesture in particular.
Like, M1 guidelines mention swiping on content to swap tabs, heck, you can even find the same on the current Material Design 3 guidelines
I think it was a conscious design decision from the Material Design team to not use that gesture in particular? Because it isn't due to conflict with other components, in the tab guidelines they call attention to be careful when the content itself is swipeable.
it's not a bad idea if you're working around gestures, but it means you can't have something where you swipe between tabs when not from the edge, and get the drawer when from the edge

or, for example, swiping to reply/forward in a messaging app, or upvote/downvote on a lemmy client
(also, subjectively, it's kind of a bit ugly)
I mean, you already can't have certain gestures with other gestures. Like you can't (or shouldn't) have swipe on a card to upvote at the same time you have swipe content to change tabs. I'd argue this restriction is better for the user because with discord's implementation it is very clear what the trigger area is, because the entire view is the trigger area.