Back in the day on TrekBBS (alas, I cannot find the original post), someone pointed out that Odo shapeshifts less and less often as the series goes on. It was never a super frequent thing, but it occurred more often in the earlier seasons, but, even accounting for his time as a solid in Season 5, he seems to shapeshift less and less in later seasons.
What reasons (in-universe and real-world) might there be for this? Was it just a budget thing? Were the writers using it as a “trick” (writing crutch) earlier on? Are we supposed to believe that Odo is trying to assimilate, or reject his Changeling heritage?
Purely from an in-universe perspective, he really seems to do it less once he learns that his people are the Founders of the Dominion. Perhaps he shapeshifts less to differentiate himself from their fascist rule, or so that few recognize him as a founder. We also see he is uncomfortable with the Vorta and Jem’hadar seeing him as a god.
This is what my head-cannon is as well. Later on during the Dominion war people were already lumping him in with the founders and because he’s a shapeshifter as well. I’d imagine when the founders are shapeshifting to infiltrate the Federation, it would make you hesitant to do so in public.
It probably doesn’t help that his shapeshifting ability is fairly explicitly part of his Founder heritage, which might make him want to use it less if he doesn’t identify with the Founders as much.