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?
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.