USE_ALT_FONTS method working on which device and FW version?

Hello,

I know there are some Hack methods there to help me add new fonts, but jailbreak becomes impossible after the FW version 5.4.5.X (I can't remember the version number accurately)

And I know the USE_ALT_FONTS method, it worked for me when I was using my KPW1.

I searched this forum for a while and didn't find a conclusive thread about USE_ALT_FONTS method, almost all the Kindle font threads are about jailbreak.

I want to know the USE_ALT_FONTS method is working on which device and which FW version. As my team and I are working on a project, to help those Kindle-newbies easily do some tricks on Kindle, so this is important to us. We don't want to release a feature that rely on jailbreak.

Thank you all