It's managed state by state. Some states have absolutely no mail-in voting, some states require you to declare a reason you can't go in-person (e.g., travel), and others will send you a ballot with no-questions-asked.

Mail-in voting is technically legal, but Republicans have been working hard to gut it and to go as far as to destroy the mail system itself. One of the few things we have left in America. In some places, it's almost impossible to mail in vote unless you prove that it's the only way you can vote to the government. So it's on par with getting disability aid. Which requires a bunch of shit, and they still normally decline cause one guy will look you up and down and go, "you seem okay to me".

The system will always preserve itself unless it’s given a little push.