With some exceptions, the only hard requirement for every macOS release since Sierra has been to have a Penryn-based CPU or higher (SSE 4.1), which has been the case for all Macs since 2008. So in theory, you could run macOS Tahoe on a 2008 MacBook. Whether you would want to though is another matter, primarily hinging on RAM capacity and CPU speed in order to support the ever-increasing number of background services and binary bloat (as outlined above).
Installing Monterey was easy enough using OCLP, and essentially the same process as installing other unofficially-supported releases such as Mojave or Catalina. And especially after the optimizations, I would say that Monterey was for the most part usable, with only minor lag and occassional stuttering during UI animations and other events, which is mostly due to the 1.6 GHz C2D, not the RAM or GPU.
My problem with it was predominantly twofold; running it came at the cost of higher temperatures and fan noise (and places unnecessary wear on those components), and I prefer to have faster performance beyond what is considered "usable" for regular use. The former might not be an issue on a desktop system of the same age or older, and the latter will differ depending on the user and their priorities, so your mileage may vary.
