Is Matt Damon fading away in terms of relevance??

Nah, he still had it in Ford v. Ferrari, the role was just more charismatic straight man than ornery character actor. His 2017 was lousy indeed, but everybody who's commented on the Last Duel test screening said the acting was good, so one person (who said the acting was great) further singling out Comer/Driver isn't enough to say Damon isn't remarkable in the movie. Darius Wolski said something praiseworthy about Damon IIRC. Stillwater being moved to July doesn't mean it's bad; perhaps not a Spotlight-esque success, but not a failure like The Cobbler either; my eyebrows would be more raised if it got a February 2022 date or something doomed like that. The general public still very much likes Damon. Maybe the Soderbergh film will be good too?

I'd more likely say he's finding his level. To me, he has always been a pretty bland actor that you could count on to be consistently good, and not much else. Not unlike Jake Gyllenhaal (who at least shows a little more ambition as an actor than Damon). In the early 2000s, he was among the top 3 actors of his generation largely because of his acclaim for Good Will Hunting and the box-office promise he showed by turning Bourne into a successful franchise. This was around the time Norton was busy sabotaging his own career and before younger actors who he had a head start on like Phoenix and Bale (that would later surpass him for their generation of actors) truly came into their own. And while Phoenix and Bale are still not bigger stars than Damon, they are considered better and more interesting actors, so they're starting to get dibs on certain prestige projects before Damon is. But more than those guys, it's the rise of Cooper in the past decade that seems to be hurting him the most. Roles that used to go to Damon when DiCaprio turned them down now seem to be falling into Cooper's lap. That, more than anything, is the most worrying sign for Damon.

With all that said, I think Damon is still doing pretty good. His 2010s has been fine.

1. The Martian - A major, unqualified success that did really well in every possible way.
2. Ford v Ferrari - A big success that did well with awards, critics, and audiences.
3. True Grit - A supporting role, but still a big success.
4. Interstellar - A mere extended cameo, but a major movie regardless.
5. Jason Bourne - Didn't meet expectations, but still a box-office hit.
6. Contagion - A movie that came and went, but it has found new life with this pandemic.

A good decade for him, even if it has seen him fall behind actors he was ahead of on casting calls.

I think many people would agree with you on this, but I personally think Damon's range has been underrated because he hasn't done a whole lot of body transformation or accentwork.  The psychology of his characters has been really varied IMO though.  I can't see Damon pulling off what they do in Walk the Line/Gladiator/The Fighter, but I also can't see Bale or Phoenix remotely as good at nailing the moral shades and charm of his work in The Departed, as both tend to be charisma blackholes, Joaquin increasingly so, and Bale decreasingly so (to his credit).  

futuretrunks and pupdurcs made great points about the fact that Damon hasn’t done a lot of body transformation, but I thought that was more due to the fact that he battled with an eating disorder and that losing weight for Courage Under Fire could have killed him. So I get why he hasn’t done something as drastic as that later on in his career especially as you could argue that maybe it didn’t do that much for him. Being in the movie probably did, but he never got any critic notices or anything, and I think the success of Good Will Hunting would have happened if he hadn’t done Courage Under Fire,

Anyhow I also side with them about his strengths. In a way he’s underrated like it seemed like his best friend was poised for the bigger career because he was working with people like Michael Bay and in these high profile relationships like Bennifer, but that didn’t pan out, and Damon came out way ahead. Then ten years after GWH, some people probably thought that the Oscars seemed to be done with him as far as acting nominations go, but so far he’s been nominated in each decade that he’s worked.

He had some downs recently, but as others have said so far he’s always found a way to come back.

He's got nothing left to prove... He has an Oscar, still very respected and worth millions.

He was never all that relevant to begin with.

The attention span of the general public is pretty long, so a few years with no notable movies is really nothing especially when Damon is such an established and renowned actor. True Grit, sci-fi Elysium, his own franchise Jason Bourne, the surprise appearance and excellent performance in Interstellar, not the mention the box office and critical hit The Martian (even though I don't understand how he is nominated for it) and also Ford v Ferrari (still co-lead even though Christian Bale shines). He has 7 films in the last decade with over 200k votes on IMDb where he plays a major role, so he hasn't really been fading anywhere.

On the note of Damon's acting, he has a recognisable screen presence and does a type of naturalism very well (see Oceans trilogy, The Martian). He's really good but imo not at the level of Bale or Phoenix yet. Of those I've seen, apart from Good Will Hunting and The Departed, there isn't much to distinguish his other performances. Most of which just fits well in the context of the film.

Aren't we all fading away?

In the wake of The Last Duel's commercial failure:

Looks like 2021 hasn't been kind to Damon, it's almost a repetition of his 2017 year.
With the double serve of commercial failures of Stillwater and The Last Duel (and despite the latter having an increased critical acclaim with the second wave of reviews post Venice, he was completely outshined by everyone in the cast, since Comer, Affleck and Driver all got best career raves), Matt Damon's star power and movie star status keeps falling into decline, and at the age of 51 years old he's not getting any younger.

It will be interesting to see if studios will still give him other chances in making and carrying big movies like these.

 your post reminded me of that scene in Fight Club


as for Matt Damon,  yeah it sure seems like he is fading away in terms of relevancy  (Stillwater and The Last Duel bombing big time prove that)   I don't know what projects he has lined up that could be good and tbh I really don't care either.  Matt Damon has always been a boring bland actor to me.  

Hehe. But their hair are darker AND greyer