Thanks for reporting it!
No staff action triggered this. So my guess is some kind of glitch must have disabled the mailinglist globally across the forum. About 20h worth of emails were probably lost.
I’ve reported the issue on the discourse meta forum.
All mailing list functionality has been re-established and hopefully new emails will start arriving soon in your inboxes. All affected users should have received a message communicating this issue.
I’m getting forum emails again. Despite getting the emails the “Enable mailing list mode” wasn’t checked. However I enabled anyway, so everything is back to normal. Thanks.
Turns out this was a deliberate decision by the discourse team. I got the following response:
Discourse now sets the value of the disable mailing list mode setting to true by default. If users on your site were using mailing list mode, setting disable mailing list mode back to false will cause mailing list emails to be sent to them again.
So basically in all discourse instances across the world, the mailinglist feature got deactivated 
. No communication whatsoever to admins about this.
This was the reasoning behind their choice:
This decision was not take lightly. It was based on extreme real world problems we saw where email costs for self hosters and other discourse installations went sky high.
We opted for a safer default, to keep costs down for people.
I am not pleased with their choice at all, especially for the lack of communication. I mean, I can’t complain much because they’re hosting us for free, and do provide a great service. But if this sort unilateral-with-no-warning of stuff happens more frequently, I will not be happy at all.
Thanks for figuring that out. I understand there reasoning, but they should have communicated that first, and even better they could/should implement “Mailing List mode” as option (opt-in or opt-out) for administrators. The forum operator should be the one who decides.