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?
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.
The VFX almost certainly isn’t cheap either.