There's probably some real old, very early, novice songwriter stuff that didn't get registered, but otherwise, I've been registering mine since the '80s.
Back then it was all hardcopy forms, and you had to send in a cassette if you were registering both song and performance/production, which is how I do it, since it doesn't cost anything extra to do both at the same time.
Back in the early 2000s. the Copyright Office started their electronic, online registration, and I volunteered to be part of the beta-test group, so I signed up for it day-one...and I've been using the electronic/online ever since.
It costs a little less than mailing in hardcopy forums and media...and it's also faster...since they get overloaded with mail, and they said it takes them as much as 6 months to even open your envelope, then you wait another 6 months to get back your registration certificates.
With the electronic filing, the whole thing takes about 6 moths. Once you set up your account, it's pretty easy, and you just upload your audio WAV files with the online form....and once you have an account, it minimizes how much you have to type in online...you just go to your account.
Anyway...it's something I like to do, it just makes the whole copyright thing official, and you get the certs back to prove it...and it brings full closure to your songwriting....not to mention, if you should ever need official proof, it's already there. Sure, the act of writing a song automatically makes it yours....but it's not legal proof. I mean, I don't find the official registration and cost that big a deal not to do it...especially if you are putting songs out on the interwebs, and releasing CDs. The way it is these days, everyone is suing everyone even stupid shit like 3 chord progressions...so, might as well have it officially registered.