the forceUpgrade option enables the new features in unrendered/ungenerated parts of the world does it not?
No, it just runs the serialized chunk data through the schema updater, i.e. DFU, and saves the data back to the disk. That process is basically what mojang uses to manipulate the serialised data of a chunk to upgrade it, i.e. renaming item types, converting old data, etc; for chunks, they'll also potentially add blocks to the chunk
loading a chunk (to a full "ticking status") in general is:
1) load data from disk
2) Run data, if it exists, through the data upgrader (X)
3) pull chunks up through the chunk system, i.e. running through the chunk generation system
4) chunk pops into the world, post processing is ran
forceUpgrade takes you up to step 2 and shoves the data back onto the disk, the retrogen stuff, i.e. below 0 chunks, is basically a weird special case where mojang basically converted the chunk status of chunks back to a lower generation so that they'd rerun through that system in order to trip the retro gen + blending system added.
So if I'm upgrading from say 1.20.1 to 1.20.2 wouldn't it be wise or necessary to run that?
No, it's never been necessary to run forceUpgrade, Mojang only ever added that because with the size of the data transformations inside of the flattening being large, as well as DFU having some sizable performance issues at first, the standard upgrade done during loading chunks was stupidly slow, hence that flag was added to just get that stage over and done with.
Outside of that huge hump, we do not, and have not, recommended using force upgrade, for the most part, it's a complete waste of time, especially with how small and fast the typical data upgrade path is these days.
