Last week-end (april the 30) i did an install of /e/OS for another user on a Pixel 5.
It is possible to downgrade from A13 to A12 ( i did it).
https://developers.google.com/android/images?hl=fr
For a unknown reason, my computer refuse to apply the “flash-all” command (I would have preferred this method), so because i was in a hurry i decided to use the online tool provided from a google serveur.
Today i found this recent xda thread that cover the topic :
https://forum.xda-developers.com/t/google-pixel-5-downgrade-to-install-e-os.4583005/