Astrophotography has been a favorite among Pixel users since its release in 2019, and it's continued to get better as Google's camera software evolves. When the Pixel 5 and 4a 5G launched with an ultra-wide camera, many were excited to see what it would be able to do in astro mode. Sadly, the quality was lacking, and now Google has disabled its use on this lens.
Once updated to Google Camera 8.1, you'll be unable to engage astro mode unless you're using the main camera. Zooming out to the ultra-wide will show a "Zoom to 1x for astrophotography" banner at the top of the screen.
It's a shame to see this restriction, although not surprising since the resulting images were so bad. Hopefully, Google will continue to work on this and bring it back someday in a more polished state. Rest assured that regular night mode still works as before with the ultra-wide lens, so you won't be missing out there.
The last update to the app rolled out around December 9, so you've probably already received it via the Play Store. If not, you can grab the latest version (or rollback to a previous one) over at APK Mirror.