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