IIRC the presence of the "brand box" on those pages is controlled by their permissions, in particular only pages running with chrome privilege have the brand box.
This is likely not a regression in Proton, because those same pages don't show the brand text in Photon or any previous themes.
https://searchfox.org/mozilla-central/source/browser/components/about/AboutRedirector.cpp#110 and if I understood correctly the pages with the badge are the ones without nsIAboutModule::URI_SAFE_FOR_UNTRUSTED_CONTENT
Based on that, we set a "chromeui" attribute here:
If we want all the pages to show it, we should probably not rely anymore on the chrome privilege and not use the "chromeui" attribute. In the past there have been discussions about showing branded pages for everything or just for chrome privileged pages.
Johann, do you remember the reasons for which we decided to show the brand badge only on _isSecureInternalUI, or reasons for which we should not extend that to every about page? I think WebExtensions cannot register new about: pages.