Currently, async support for SPI (and implementation of the embedded-hal-async traits) is dependent on CONFIG_SPI_MASTER_ISR_IN_IRAM being disabled:

There are two things:

• The crate documentation doesn't make this obvious at all. One has to dig into the source to figure out why read_async and co aren't defined and figure out that it's a IDF setting.
• Async SPI support should still be provided with the interrupt handler in IRAM.

The first point is pretty straightforward to address, but addressing the second would make that irrelevant - not sure how much effort the second point actually is.