MID-s are used by MUA-s for referring earlier messages, tracking answers and so on. So any software expecting dialog (messages coming back) needs to deal with MID-s correctly. Missing MID-s show that said communication is one direction, because broken dialog has not been problem.

My take, as a postmaster for hosting company, who don't have any sympathy to gmail (that should be visible from my comments history): Message-ID is absolutely MUST in production e-mails. You can send your test stuff without it, but real messages always have it. Not having Message-ID's causes lot of fun things. All somewhat competent software is capable to add Message-ID's, so lack of it is good indication of poorly made custom (usually spamming) solution.

Rspamd and spamassassin have missing MID check in their default rules, I am sure that most antispam software is same.

Seems like supporting argument against governmental regulation. In this specific case, against sanctions.

Testing with different ISP-s involved ISP provided resolvers and directly using archive.today nameservers. I know that archive.today has previously blocked common public resolvers, like 1.1.1.1. That case they replied with 127.0.0.4 or other loopback IP addresses.