Square's Avengers game not showing up at The Game Awards was honestly a sign (to me) that development may be rocky.
This teaser was almost two whole years ago, and it came with the explicit statement that we'd hear more about the game in 2018 (which already felt like a long time to be waiting). 2018 has instead come and gone without so much as a press release. Games have gone longer than that between first announcement and first actual details, but it's pretty rare. And what makes it more concerning is that their statement of getting more information in 2018 was a pretty low bar and they still managed to stumble over it. Total radio silence. I'd be very interested to find out what's going on behind the scenes at Crystal Dynamics.
I don't know that this is necessarily some massive, dramatic story. It's possible that development simply hadn't even started when the first teaser hit, and it took longer than expected to nail down details on what the game would actually be and start production. It's possible that the initial concept of the game ran into some roadblocks and it's going through some reworking. These things can be overcome in time. But I do think that this is a sign that the project has not been on a smooth development track so far, and it may be a while longer before anything comes of this.
EDIT: Or, hell, there's the simple answer that they decided to move it to next-gen and nobody wants to really talk about PS5 titles before the machine is announced next year.