My Pixel XL stopped receiving text messages on 5/24/2020 (9 days ago).
I tried restarting it, disabling wi-fi, uninstalling/reinstalling all Messages (default google messaging app) updates, disabled bluetooth. I even sent a text message to myself and never received it. It had been several days and nothing worked. My phone has had 50% free space, and my plan has unlimited texting and data. I could send/receive phone calls without issue, and other people told me they were receiving text messages that I sent out, but I was not receiving replies.
I just now tried marking all messages as Read.
Turned off "Mobile Data" on the phone.
And then sent a text message to myself.
The phone suddenly received all missing text messages, including MMS group messages, since May 24th, but complained that it could not send MMS with mobile data off. (The message I just tried to send myself?) I've turned Mobile data back on and I'm receiving new messages again.
I suspect that turning off Mobile data is what finally fixed it, but maybe I had too many unread messages?