The Flatpak is already packaged and works well. It just needs to be maintained from a person that joins the Inkscape community.
This would allow further improvements like Portal support and making the app official on Flathub.
Update: One might have been found!
Are all Appimages using that, if not what percentage of the ones you know?
Usually if the appimage has a github release with a zsync you have that verification.
And are tools like Gearlever enforcing or using that signature check?
I don’t use gearlever, as far as I know gearlever doesn’t even let you sandbox the appimage like AM does. I don’t think any of those forces signature verification besides AppImageUpdateTool and that’s because that’s part of the zsync update process.