Yeah, I also saw the news on Rhodox's that he got busy with life. I figured that was what happened however I am glad to hear there he is okay! I hope one day he'll be able to turn his attention back to Painterly. I am quite fond of Painterly and have been a fan for a long time. Does anyone remember that texturing livestream donation event he did a couple of years back? That was pretty fun.
And Taiga, the customizer is only for vanilla. However, you can download "developer version" of the texture pack and there's a folder full of alternative textures. If you've the patience to sort through them all, you can use them for mods I guess.
Also, the official website is down again. Given the lack of interest from the original creator and the community, is it safe to say support for the pack is now dead?
The website is not down, at least not anymore if it ever was. And considering "it's unlikely I have time to properly devote to Painterly", probably... at least for now, maybe he will pick it up in the future, or someone else will.