Hello Marc,
the RGB565A8 color format means that a bitmap consists of one memory area containing the 16bit per pixel RGB565 values and another memory areay containing the 8 bit per pixel Alpha values.
This format was introduced many many years ago for one hardware that was only able to operate with two different memory areas.
In principle, your proposal to use RGB565A8 instead of RGBA8888 for bitmap resources is a good idea to reduce the footprint. Unfortunately, this format is not advantegous for software drawing routines and it is not supported by most hardware graphics accelerators.
You already found parts of version 9.10 documentation online - this version will be released next week. With version 9.10 the Platform Package and Build Environments for NXP i.MX RT1050 and RT1060 will use the pixel pipeline hardware (PXP) in order to accelerate fill / copy / blend operations. This would be not possible with RGB565A8 resources.
Btw: We are just discussing to treat RGB565A8 and RGB555A8 as deprecated.
Best regards,
Manfred.