SPI allows word sizes of several bits, not necessarily rounded up to a multiple of 8.
While I don't now about every platform / hardware, at least soft-SPI (GPIO bitbanged SPI) and the ESP IDF driven ESP32 SPI hardware support transferring single bits via SPI.
Looking at the micropython code though, the whole SPI abstraction layer - HW backed or SW - assumes lengths of multiples of bytes, meaning, I can't just adjust the parts related to esp32 / gpio.

To provide a use case - and I indeed see this being an edge case, but keep in mind that it still conforms with the SPI spec: (ab)using SPI to speak SWD.

Suggestion / Feature request: Adjust the SPI code so that we can transmit single bits instead of multiples of bytes.

Happy to do it myself / help / assist / for discussion. However at first glance, the changeset apparently would be larger than anticipated and I'd be glad for opinions / input / help.