A few years ago I mistakenly bought some used ECC DDR3, 256GB (8x32GB) of the stuff for $110. Given the current memory shortage, is it time to dust it off and let someone else put it to use somehow? Or is DDR3 just too old to be worth it? I already have 128GB of DDR4 in my server and don't really need more for myself, but I have considered getting a second server to play with that I can stick these in.
It isn't necessarily a 10-15yo server. Intel was releasing CPUs with DDR3 support as late as 2017-2018 (Goldmont/Denverton/Apollo Lake arch), and many of those embedded CPUs went into low power servers and mini PCs.
Sure, but those won't take registered ECC RAM. Only full fat server CPUs will.
Some desktop ones do. I think the motherboard also needs to support it.
Registered and ECC are separate details. Some desktop boards accept ECC (not very many Intel), but none accept registered. I'm unclear on what the OP has since they only mentioned ECC and I didn't Google the models.