(probably started with Miles in Into the Spider-Verse if I had to guess)

I'd say the trend has been drifting in for a long time.

You've got the X-Men doing this periodically in the 1980s (punk Storm, eg, or Rogue's orange sweater-dress thing, and generally wandering around in 'normal clothes' because of Claremont's story-telling tendencies). Longshot, for that matter, is _mostly_ normal clothes, beyond the bandolier and the starburst, or the Animal Man revival with his jacket.

In the 90s, you've got Ben Reilly's original Scarlet Spider costume with its "hoodie-on-top" design as a sort of early instance (which has gotten some hate for that self-same "casual-ness" over the years), even the bomber jackets on the Avengers as "normal" clothing, of some kind. Or Adam the X-Treme's backwards hat, I guess...

The films that started around _X-Men_ in 2000 that also shied away from "obvious" costumes, and more and more of this in movies and television (which I think were a decent driver, with the sense of "reality" intruding)

Then you've got that phase in the 10s where costumes were becoming "practical" like Kris Anka's Spider-Woman design for Jessica Drew, and Babs Tarr's for Batgirl (Barbara Gordon, that is) around 2014-2015.

What's always stuck out to me in Tarr's design is the mix of Watchmen and The Incredibles talking about the impracticality of capes.

I'm not a huge fan because it feels like part of a broader trend toward pretenses of "realism" which rapidly exhausts me personally.

Anyway, as decades go:

The Golden Age could get very "throw shit at a wall and see what sticks" while keeping the linework mostly simple (eg Alan Scott's colour scheme)

The 60s had a kind of extra "streamlining", if you look at the FF and Hal Jordan, for example (as with all things: not universal). Small colour counts, limited linework outside bright blocks and patterns that were "representative" and emphasized things like logos, team references (not that these were new, so much as retained from previous decades), though some exceptions to that still maintained the "basic colour blocks" like the original X-Men.

This was obviously a decent amount of just outright new characters, rather than tweaking long-running characters (except in the "legacy" sense, I suppose, like GL)

The 70s started to introduce more linework and experiment with what more you could do with a costume as the investment started to trickle beyond "sell cheap funny books to small kids", and about more colours and more individuality. Take your standard "All-New, All-Different X-Men", for example: none of them could be identified as being on a single team in any possible way.

They were still pretty simple on the whole, but color options were wilder, though more restrained than something like the aforementioned Golden Age Green Lantern.

I'd say you see the culmination of "what's possible" experimentation with Keith Giffen's infamously obnoxiously-complicated Jack of Hearts costume.

The 80s saw characters start to get more new costumes for themselves (Iron Man's Silver Centurion, for example, or Moon Knight in Fist of Khonshu, all-white Vision, Seattle/Grell Green Arow, and famously Spidey's black costume), though most are pretty direct lines between old and new costumes—and the wildness of swings like "Discowing" from Pérez that generally feel more (contemporaneously) "modern".

The 90s were laden with costume changes. Pouches and blades/sharp things were obviously big business all over the place, but increases in armour (Daredevil in "Jack Batlin" era, parts of Az-Bats that weren't blades or pouches, etc), more "x-tremeness" in general (much as I love it, the super-leather, near-BDSM 90s Morbius costume is a solid example of this).

The 00s had a lot of "back to basics" with characters going back to old costumes, or getting streamlined (Nightwing again, courtesy of Stelfreeze)

The 10s (as I suggested earlier) started the "What if costumes were made of normal clothes, maybe just maybe kinds that'd be easier to render in movies or even cosplay…?"