This was first the case in 1.7 (not a typo), but is even worse in the Caves and Cliffs development datapack.*

Ever since the world generation was changed in 1.7, caves were significantly nerfed. This makes dungeons also significantly nerfed.

An example of this is the fact that I have played Minecraft for hours on end and found exactly 1 double dungeon ever (where a dungeon generates on top of another), and that being with the help of an outside source (Chunkbase), and I had to play in 1.16.5 (Actually, I checked later on and it wasn't technically a double dungeon, just two dungeons that generated about 30 blocks away from each other, so that doesn't even count). However, upon loading a new world in 1.0.0, I search around for about 5 minutes and find a double dungeon (recall this is before spectator mode). This is evidence that dungeons were much more common in the past, a staple of caving. Now they are incredibly rare (in 1.7-1.16.5 they are about 2.5x rarer than in 1.6 and before).

In addition in 1.7, they changed the y-coordinates that dungeons can generate from 0-127 to 0-255, which since caves rarely generate above 127, majorly reduced dungeon generation unintentionally, making them in a default world about half as common. In the Caves and Cliffs development datapack, they are even rarer, being that it searches from -64 to either 255 or the build limit, I'm not entirely sure.

(2.5x rarer comes from a number I read that said caves are about 23% rarer and the limit that dungeons can generate has been doubled. 23% ~= 1-.80, .80^-1 = 5/4 = 1.25, 1.25 * 2 = 2.5)

*I've done some more research, and it is possible that dungeons started to get rarer even before 1.7. Nonetheless, I was able to quickly find a double dungeon in 1.6 (not a typo).