As you say http://www.espruino.com/Reference generally lists every function and where it is available so you can work it out from there.
What do you mean by 'low flash devices' though?
There is no pre-made list as every device is different, but if you're willing to check out the Espruino repository you can do BOARD=YOUR_BOARD_ID make docs and it'll make a reference file of everything that is available for that board and nothing more
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
As you say http://www.espruino.com/Reference generally lists every function and where it is available so you can work it out from there.
What do you mean by 'low flash devices' though?
There is no pre-made list as every device is different, but if you're willing to check out the Espruino repository you can do
BOARD=YOUR_BOARD_ID make docs
and it'll make a reference file of everything that is available for that board and nothing more