Most of the RAM that I've seen listed on this thread has been stated as being registered. Are all the listings generally inaccurate in that regard?
Which listings specifically are you referring to? Most people here bought the kits from ADATA, Kingston, or Innodisk. I haven't seen those advertised as "Registered" yet myself. All of those are Unbuffered, only some (like the Innodisk, Crucial and V-Color kits) are Unbuffered ECC. Registered DIMMs won't work on virtually all consumer boards.
Tell us what kind of memory you're looking for, maybe I can help you. I got a list of links somewhere
I believe that the v-color sticks that sometimes get mentioned are incorrectly listed in some places as registered, but in fact are not. There are many ecc udimms that are listed, which should work correctly with decent amd boards (with ecc enabled), and probably should run fine on intel boards with ecc not enabled (since intel reserve support for ecc to their xeon chips, or at least they did).
I have also seen bad recommentdations from others, and there is an optimum tech video (of the black ridge cooler, iirc) that links to vlp rdimms that won't work for the vast majority of consumer boards, so it is a mistake or oversight that is often made.
Registered memory is reserved for servers, typically. Support is required from the motherboard as much as it is from the cpu chip itself. The reason you would want to use it is to have a higher capacity of ram, since it doesn't stress the memory controller as much. On the AMD side of things, it's something that you'll only find on the epyc stuff, as it isn't supported on threadripper. I'm not sure about intel, but it almost certainly isn't available on the standard i7/i9 series of cpus and their motherboards.
tl;dr : avoid registered memory. ecc unregistered dimms work with most normal boards, registered memory will not.