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.
I think narratively it’s just a much more enjoyable to not have Odo shift shape onscreen, especially as it gives him the ability to overpower a lot of people. In the early episodes this needed to be shown so we could understand Odo’s character, but by the fourth season (both when he was a solid and not) we knew enough about the Changelings that we didn’t need to show this as frequently and had to figure out more creative ways to to get around Odo.
My bet on the real reason was it’s boring and solves too many problems. It also likely cost more than just having the actors do something instead.
There’s probably lore reasons you can use, like the one mentioned here about trying to differentiate himself from his ancestors, but I’d bet the real reason is just for better storytelling opportunities.
I remember one scene where our heroes are fighting Jem Hadar in hand to hand combat and Odo is there (with his shapeshifting abilities, not during the arc where he’s stuck as a solid). Odo is punching people in the face and stuff. I remember thinking, couldn’t he just morph into a buzz saw and spin around? Or turn into an octopus with eight terminator-like hand swords? Or maybe a boa constrictor and wrap around them and squeeze 'em all to death? Or…