I'm of the mindset that there are more monsters out there than I'll ever use. But it hasn't stopped me from accumulating so, so many of the books (including all the Kobold Press Tomes and Creature Codex as well as MCDM's Flee, Mortals!). I'll tweak monsters here and there but I've never felt the need to create one from scratch so these "missing" rules don't move the needle of concern for our table. I also don't worry about "balanced" combat encounters, but that's another thread topic all together.
Now, for those who really
need those monster creation rules, I say go get Sly Flourish's
Forge of Foes. If nothing else, it's a good read and, as mentioned upthread, stands up well with the 2024 rule set.
Not sure you want to buy it? Go to that
link anyway and get 30 pages for free...