CalyxOS has decent Pixel 4a support, in my experience. I bought my 4a last December specifically because it was supported (and the right price on Ebay). Install was pretty straightforward, and they don't have as stringent a support cycle as GrapheneOS, although I did not try installing that one.
I'm guessing the answer is mostly no, since GrapheneOS was the most Pixel-friendly AOSP project I knew of, and they're also "buy a new phone, n00b" https://grapheneos.org/faq#legacy-devices
[+] [-] forbiddenlake|2 years ago|reply
https://support.google.com/pixelphone/answer/4457705
[+] [-] nightshadetrie|2 years ago|reply
[+] [-] stringtoint|2 years ago|reply
[+] [-] sach1|2 years ago|reply
[+] [-] irq-1|2 years ago|reply
[+] [-] mdaniel|2 years ago|reply
https://doc.e.foundation/devices and https://wiki.lineageos.org/devices/#google both list quite a few of them, but unknown if it's that they are currently available but once the blobs get locked it'll be the same story: unsupported
And, of course, going with "community" distros also nuke Play services and thus almost certainly will not allow Google Pay or other niceties