It turns out that when you delete your Google accounts, Google unsubscribes you from any (public and private) Google Groups you're a member of. I found out about this only because my inbox traffic these past couple of days felt unusually light so I went and looked at racket-users and, lo and behold, there were a bunch of new posts I hadn't received.
I get it. They want to avoid sending emails to an address that, from their perspective, no longer exists. Makes sense, although they could certainly tell you that you're gonna be unsubscribed when you cancel, maybe even throw in a list of all the groups you're currently subscribed to for good measure.
What annoys me, though, is how hard they make it for someone who
doesn't have a Google account to join a Google Group. If you visit a
group and you're not logged in, there's no way for you to join via
the UI. This makes it seem like you can't join unless you create
an account, but that's not the case! You can still join groups by
sending an e-mail to group-name+subscribe@googlegroups.com. For
example, to join the racket-users group, you would send an e-mail to
racket-users+subscribe@googlegroups.com. If the group you want to join
has spaces in its name, replace the spaces with dashes in the e-mail
address.
If you were wondering about how to join a group without a Google account, now you know! And if you're considering starting a new mailing list, I urge you to take this factor into consideration and maybe use something like groups.io or lists.sr.ht instead.