We gave up manually creating the checksum file because GitHub now creates sha256 checksums for all assets uploaded to releases automatically (as you noticed). I purposely removed the latest checksum file link from the website since it would result in a broken link anyway.
I'm not going to try and duplicate what GitHub is doing with the button to copy the sha256sum, because I frankly don't think it's possible--at least not in an automated way that I won't have to manually fuss with for each and every new release. If I find a way to automatically pull the checksum from GitHub, I may consider adding it to individual releases, but I doubt it will ever be part of the Latest Downloads Page again.
In the meanwhile, I will remove the reference to the CHECKSUM.sha256.txt file from the website (since it's not going to be a part of our releases anymore) and recommend they go to the GitHub release page to get a sha256 checksum if they so desire.
Much like the user guides, user contributions are appreciated for the website. The Sigil/PageEdit website is actually automatically generated from markup stored in a github repository. Folks who want to see changes to the website should consider creating a pull request to help out.
The repo that hosts the website's content is:
https://github.com/Sigil-Ebook/sigil-ebook.github.io
The markup file that is used to specifically create the
https://sigil-ebook.com/sigil/downloads page is:
https://github.com/Sigil-Ebook/sigil...oad.md?plain=1