STM32F042x6 series cannot do both USB+CAN+send/receive about candlelight_fw

[fenugrec]

Isn't it true for the STM32F042x6 series too?

No. I have boards with a F042C6T6 (lqfp48) working, both TX+RX. The source you cite is only right for smaller packages <= 32 pin I believe.

But AFAICT it's game over for this IC/package if I want to have a functional send/receive USB+CAN board, right?

Correct.

Perhaps it should be stated in the README alongside the STM32F103 remark?

That remark was mainly because we were receiving many questions about the F103, and that IC's limitation (shared buffer mem) is really not obvious when skimming over the datasheet ( IIRC it's only mentioned in 1 place in the RM).
I'd argue the F042 case is more evident : just looking at the pin assignments shows that only the 48-pin variant will work.

[fenugrec]

I added a comment in 68df7d5 .