so this is my take, as a developer (not connected to percussa) :
at its heart the SSP is a linux based arm ‘computer’ (or more an embedded SoC really).
so given JUCE supports arm/linux, the SSP can use anything that the juce sdk supports on linux.
Synthor is a VST host this means you could develop the VST with what ever software you liked, not just JUCE e.g. you could use the lower level steinberg sdk directly for vst integration, and directly access graphics, filesystems etc.
juce is talked about (and example supplied ) because JUCE is a very popular sdk for plugin development since it simplifies development (*).
similarly the ‘recommend’ version of juce in the sdk instructions, just aligns to what is being used for synthor development, but you could use a different version if you wish. (**)
(*) note : there are licensing restrictions on JUCE… so if you are planning on selling a plugin you would be wise to look this up - and consider if juce is still your best choice.
(**) Id stick to version used by synthor for simplicity, but be aware online documentation on juce website is latest version - though the beauty of juce is you have its source code to grok!
so the percussa sdk is not really juce… nor id say something percussa ‘supports’ or not.
what is the sdk then?
its simply a ‘declaration’ that synthor will host VST plugins, and which parts of the VST api are used, and how these relate to the hardware.
in particular, how parameters are used to send information about the hardware, and its state within synthor.
I guess the other side of the ‘sdk’ is an example, which shows how the above works in practice.
for help on juce, Id say the juce forum is excellent.
you will also find a ton of open source plugins developed using juce that you can use for inspiration etc.
there’s little point in repeating this here… few users are interested or experienced in Juce (or plugin development) here - whereas the Juce forum is 100% dedicated to this and has Juce employees responding to questions - so best to get information/help from the ‘horses mouth’ so to speak!
again, nothing really specific to the SSP here…
one thing you may need to be careful of is that juce is often a wrapper around audio formats, and you need to compile support in, also this may require dependent libraries.
(obviously be careful of using dynamic libs that are not already installed on the SSP as this may give issues for other ssp users when you come to distribute it)
again… this is all covered in the Juce documentation, i think also in the tutorials - and the you’ll find discussion on the forum.