[azernik]

Mandatory deposit applies regardless of registration; it's just that registration requires sending a copy, which satisfies the mandatory deposit rules.

At the Copyright Office FAQ (https://www.copyright.gov/help/faq/mandatory_deposit.html)

"Mandatory deposit (17 U.S.C. section 407) requires the owner of copyright or the exclusive right of distribution to deposit in the Copyright Office for the use of the Library of Congress two complete copies of the best edition within 3 months after a work is published. Section 408 of the copyright law, for a fee, provides the option to formally register the work with the U.S. Copyright Office. This registration process provides a legal record of copyright ownership as well as additional legal benefits in cases of infringement. Optional registration fulfills mandatory deposit requirements."

Then later:

"Yes. Under certain circumstances, special relief from deposit requirements may be granted. The grant of special relief is discretionary with the U.S. Copyright Office and will depend on a careful balance of the acquisition policies of the Library of Congress, the examining requirements of the Copyright Office (if registration is sought), and the hardship to the copyright owner."

Note that "if registration is sought" - even if you're not registering, you still need to apply for an exemption.

[lsc]

https://www.law.cornell.edu/uscode/text/17/407

"Neither the deposit requirements of this subsection nor the acquisition provisions of subsection (e) are conditions of copyright protection."

Which is to say, of course, that you are right, however you don't lose copyright protection if you don't comply. - it looks like there are no penalties until they ask you for a copy.

This lines up with my understanding that if you are serious business, the LOC gets a copy. If not? it doesn't really matter, and if you become a big deal later, well, following the rules helps you establish provenance, but isn't required for protection.