Pretty much all Allwinner based SoCs used in common SBCs have FOSS firmware. Some older Rochcip SoCs also do, but you have to be careful there (the most open/fully featured is RK3399). That covers a huge amount of SBCs out there. Some even have multiple alternatives for FOSS firmware.

If anything RPi seems like an outlier. Out of 9 different SBCs models I have at home only Rpi2b and Quartz64-A have proprietary firmware.

Some RK3399 boards can be used AFAIK entirely blob free. This has been true for a few years actually: https://stikonas.eu/wordpress/2019/09/15/blobless-boot-with-...

The Pine64 guys are working towards that goal I think. You can flash open source firmware on some of their boards, but I don't know if it's the default.

I’m not an expert, but iirc banana-pi and beagleboard are SBCs with more of an open source bent.

Beaglebone