Great question! We have a Buying Guide for visual novels which tells you where you can find DRM-Free visual novels to the best of our knowledge.

We cover JAST in this section: https://wiki.comfysnug.space/doku.php?id=visualnovel:buying#localizer_stores

Founded in 1998 and has a firm stance against DRM since 2010. Publishes games from several other localizers. Offers DRM-removal patches for their pre-2010 games. You’ll see this in the Disclaimer tab for every game: “The software is DRM FREE. No internet connection is required to play. You are free to use and copy the software for your personal needs without restriction. Additionally, we don’t require authentication software, because that’s lame. You shouldn’t need to give up your privacy just to have fun.”

I recently bought two visual novels from JAST, and they give you only the game files. You don’t need to install a game client to play them, and I don’t think JAST has a client. The visual novels worked fine through WINE on my GNU/Linux system.