A feature I often use in Adobe apps is the Proof Setup option. In InDesign this allows me to check how document colours will look across different print and screen colour profiles, including some greyscale profiles. In Illustrator and Photoshop this menu option also checks how document colours would appear to someone with Protanopia or Deuteranopia-type colourblindness (for some reason this feature isn’t in InDesign).
The colour profile previewing is nice to have, but I usually focus on just using the greyscale profiles as a way to test contrast in my artwork colours, and in Illustrator I often like to check that I’m not using colours that wouldn’t read well for a colourblind person.
I’d love for Affinity apps to include greyscale and colourblindness preview options, as well as any other preview modes for visual impairments that may be useful. Having the ability to proof specific colour profiles would also be useful.