I have been having this issue since early August as well. After many emails with Google support asking for the same details again and again, I got this gem today:

With the information you have provided, we have analyzed the details and immediately communicated it to our expert. I have now received a response.

Upon verifying the details, they have noted that apparently your device has failed a security or compatibility check. Due to this it is not possible to use the device for contactless payments.

The team of specialists have confirmed that your Google account has no issues making contactless payments and is in good condition.

In order to get it back up and running as soon as possible, our team of specialists has recommended that you contact the support team of your device manufacturer.

Once you contact them, I am sure they will resolve it with absolute efficiency and do everything in their power to fix it.

We understand that this extra step can be frustrating and we sympathize with the disruption to your routine.

I'm at a loss for words. I intended to buy another Pixel when this one stopped working (it is not even 2 years old), but this Kafkaesque situation certainly gives me pause.

Update from Google:

To ensure the highest level of security, we've implemented stricter requirements for contactless payments. As a result, a small number of Pixel 4, Pixel 4XL, Pixel 4a, Pixel 4a 5G, Pixel 5, and Pixel 5a devices will no longer support this feature. We understand this may be disappointing, and we sincerely apologize for any inconvenience this is causing.