The Avengers team adopted this as one of their methods of releasing new characters last year with the release of Hawkeye and Kate. It's a way to ease the pain of fans waiting so long for content.
This isn't true, kate and clint were supposed to release very quickly after each other and they were going to tie-in with the hawkeye disney+ show (their movesets are also completely different, so not an "echo" in the smash terminology), the show got delayed and the delay for kate and clint followed until a firm release date for the show got announced.
They already worked on them however and there wasn't anything else now they could release, so kate released after a month delay and clint got padded out to a few months later.
Black Widow then also got delayed, so they also delayed the red room content until it could tie-in with the movie; the black panther content was worked on first, but got delayed as well because of the death of chadwick boseman.
Spider-Man was always announced for "2021", but the movie was delayed to november 2021, and then a final delay to december, so there was nothing to tie-in the release of the character for months.
And now they suddenly announce Jane Foster, a day after the Love and Thunder trailer drops, and it seems to release in the same month as well, for another tie-in; they are clearly beholden to disney at some level, for tie-ins, and a smaller team + covid completely destroyed any content plans they initially had and it feels like they are just going from tie-in to tie-in now.
I don't think we can say they "adopted" "echo" characters until jane's gameplay is revealed, since clint wasn't an "echo" of kate.
With she-hulk being the next rumoured character, it would also be very weird if she was an "echo" for hulk.