Because of the lack of support, the idea of having udders is an assumed "no". In my opinion, if there is substantial quality work behind it, I think it would be something worth considering, even if it's just niche content. As long as it doesn't go against the rules, it should be okay.
If udder organs are to ever be implemented, you would have to figure out if it would be a purely cosmetic body part (like antennae, genital slit, etc.) or if it would be fully functional (essentially be its own breast class object with variables, most likely connected to the current lactation system or have its own independently). In either case, the udders would need several essentials to implement: appearance blurbs, adjective lists and word banks, any tracked stats, etc.--but those should be easy; the main issue is whether or not there would be written content to support it. Will the player character and the NPCs acknowledge it? Will certain scenes mention it? Will there be scenes revolving around it? So, similar to genital slits, it may have very little to no support going in and it will ultimately be up to the authors who want to check for it at all.
If you want to create an udder-making item, by all means, go for it. Just expect it to not have much support unless a writer has a fetish for it or you are willing to write in some of the connecting content/scenes yourself.