I do not want to have a bunch of addresses on one line with commas between them.

That's fine. There are just a bunch of addresses on one line with no commas between them. Also see https://support.mozilla.org/en-US/kb/addressing-email

I want a separate line per address, as per the previous behavior.

You can't.

So, my only solution will be to revert to previous version of the software (which won't be seeing security updates), or start using different software. Thanks for the non-answer anyway.

Maybe one day, probably long after I'm dead, other developers will learn to not force UI/UX changes on users, without options to continue to use their preferences. I've always offered UI reversion options in my applications updates, it is not that hard to accomplish, merely requires actually caring about your users.

That is a business choice that you are welcome of course do decide for yourself for your own code. We however cannot afford to arbitrarily keep large pieces (we're not talking about a few lines of code) of ten year old code throughout the code base that has been superseded. It's not a sustainable model, and long term not in the best interest of our users.