What dev board you use has no bearing on the firmware you can use for the ESP8266 and you can flash it for whatever firmware you want as long as there is enough flash memory, be it NodeMCU (Lua), Arduino, etc (they're all built on top of the native Espressif frameworks, regardless). All non-trivial firmware on the ESP8266 requires binary blobs from Espressif to run, however.
You're not forced to use it and it's not a blob[1].
Just looking at that briefly it looks like there are still closed source components. (although they might not be part of the RTOS so I might have been wrong about that)
Re:The Untold History of Arduino (2016)
by inamberclad on Sunday March 17, 2019 @09:38PM
(#19418068)
I think there's a decent re-implementation out there but as far as I'm aware most of the mainstream software for the Espressif chips still uses the blobs.
Re:The Untold History of Arduino (2016)
by ficklepickle on Sunday March 17, 2019 @03:14PM
(#19415838)
You can even use the arduino IDE thanks to the awesome work of this [0] project.
I don't like having blobs on my laptop much less in MCU firmware.