From what I've heard only about 4% of CoD: BO6 buyers were on PS4. It might finally be getting to the point where it just no longer makes sense to craft an entirely different version for the older consoles. Perhaps Switch 2 getting CoD will keep the PS4 version on life support however.
I wonder how much effort it really takes to release a game for both PS4 and PS5, especially if you're already building it for Xbox and PC. PC requires you to support a wide array of performance capabilities, at least in theory making it easy to scale back performance to a previous-gen console; and they're probably still using an evolution of the same engine they were using for PS4, so at least to my mind it's a checkbox, some performance tuning and a bunch of QA. (at least in theory). maybe some different servers to support multiplayer - but since CoD supports cross play maybe not even that.
Agreed, and 4 years into PS5, the onus on making the PS4 a "quality" experience is lower, vs. just giving PS4 owners "something" to play e.g. in developing markets where people mightn't have upgraded yet.
An analogy I might draw is the FIFA games, where FIFA 14 came out on the PS4 and PS3, but also the PS2 and Wii, which were just roster updates of previous years (no new gameplay features whatsoever), and clearly that was acceptable to enough people to give EA the trouble of developing, printing and distributing.