It looks like some have DRM and some don’t: https://bookshop.org/info/ebooks

What is DRM? What is a DRM-free title?

• DRM stands for Digital Rights Management and is the term for the anti-piracy protections most publishers require to be applied to their ebooks.
• We currently don’t support a method of transferring DRM titles onto devices with proprietary operating systems and app stores like the Kindle or Nook.
• Certain publishers, like Tor, provide their ebook titles DRM free, making it possible for us to allow our users to download and transfer those ebook files to non-Android or Apple devices like the Kindle or Nook.
• DRM-free titles can be downloaded and manually side-loaded onto your preferred device. Please refer to your device manufacturer’s support documentation for side-loading instructions.

How do I download and transfer DRM-free titles?

App

• DRM-free titles are available to download from your app’s Ebook Library by tapping the three dot menu under the ebook cover image, then selecting Download / Transfer.
• You can also download DRM-free titles from their product page by tapping the “Share” button at the top left of the screen, then selecting the Download / Transfer option.

Browser

• DRM-free titles are available to download from your Ebook Library on the website by clicking or tapping on the three dot menu under the ebook cover image, then selecting Download / Transfer.
• You will find the same button on the product pages of owned DRM-free titles as well.

This sounded great until the part where you can only read the ebooks using their app and you can’t transfer to kobo/kindle. That is a dealbreaker for me unfortunately.