Three things:
1) Giroux has had a lot of bad puck luck, but when it's this bad, maybe his passes aren't as on point as in the past, that is, all it takes to reduce the shooting of his passing target is being subtly off target, they have to hesitate, adjust and shots are wide or blocked, don't know if there's a stat for that, but the drastic falloff is odd. Remember, we're talking a player who was an excellent passer, so just a decline to "good" would have a significant impact, but nothing like what we seeing. So it's probably mostly a fluke.
2) Giroux has lost some speed over the past few years, he had a bounce back in 2017-18, was still good last year and hasn't "lost" it this year, rather, he's had to play more conservatively as he slows down, especially at center. Pretending he's the same player at 32 that he was at 26 is wishful thinking. One thing about age curves, players don't "follow the curve," they tend to fluctuate around it, so 2017-18 was probably above the curve, this season is below the curve, next season? He'll be 34 in the last year of his deal, not ancient but no spring chicken either.
3) Giroux is still a top player with Couts at center, probably because he's asked to do less and can use the skills that he has to best advantage. Watching him with TK and JVR, he's more a passenger, letting those two lead the rush and playing a more conservative role - which is not necessarily a bad thing. TK has taken a step up this season and can drive that line on offense, so Giroux can play closer to the blue line and be more of a facilitator and be able to get back on defense at center.
Managing aging veterans is a matter of putting them in position to succeed and not asking them to do the things they did ten years before.
Ideally, when Frost is ready/Patrick returns Giroux can return to LW with Couts.