> However the reality is that while modern work is still in copyright regardless of registration status, in case there's a legal suit the possible remedies are severely curtailed if it's not a registered work, so to this day it's still effectively mandatory

It should be noted that only some of the possible remedies are curtailed. Attorney fees and statutory damages are curtailed, but injunctions and actual damages are not.

Also the curtailing is just for infringement before registration. I'm not quite sure what that the implications are for that.

For infringement that has stopped by the time you register and sue it seems straightforward--all you might get are actual damages from that infringement and an injunction ordering them to not infringe again.

But infringements are usually ongoing, I believe. So what if you just sue for an infringement that happened after you registered? Can you then get attorney fees and statutory damages?

If you can, then in the case of ongoing infringement it might not make much difference. Your attorney costs are going to be about the same regardless of whether the case is just dealing with the infringements after registration or is deal with all of the infringements. And statutory damages are per work infringed, not per infringing copy, although I'd expect the amount to be lower if they are only meant for post-registration infringement.

The statute that curtails attorney fees and statutory damages pre-registration says it applies to "any infringement of copyright commenced after first publication of the work and before the effective date of its registration [...]". The question then is when does an infringement "commence"? Would ongoing infringement all count as commencing with the first infringement, so it would all be denied attorney fees and statutory damages?

There's nothing I noticed in the definitions section of the copyright statute that says what "commence" means. I did find that for purposes of the statute of limitations on bringing a copyright suite, the time is counted from the most recent infringement that is part of an ongoing infringement, and would guess that it has similar meaning elsewhere but law is not always that consistent.

> This... has been a grumbling of some countries since that Berne convention requires that there should not have formalities, and this seems to retain that formality system

The requirement that you register for suing only applies to infringement of works that are "United States" works. A published work is a "United States" work only if it was first publication was in the US or simultaneous in the US and somewhere else, or if it was first published someplace that is not a party with the US in any international copyright agreement and all the authors are "nationals, domiciliaries, or habitual residents of, or in the case of an audiovisual work legal entities with headquarters in" the US.

So, if you are say some author in some random Berne country, publish your work there but not simultaneously here, and then later find someone in the US infringing, you don't have to register to sue here because that would not be a "United States" work. The US argues that this satisfies Berne. Berne is about how you treat works and authors from other countries. It's OK if you treat domestic works and authors worse than foreign works and authors.

You do need registration for attorney fees and statutory damages, but I think the argument for that being OK under Berne is that the only remedy Berne requires is the seizure of infringing goods.