We're not suggesting moving away from the HTTPS protocol. Gmail and other web email apps, as well as Word Online etc. still use HTTPS to communicate with their backend infrastructure. They are just registered in your browser as apps that can handle the mailto:// or ms-word:// URL schema. This registering most likely happens automatically when you have visited a page that supports the schema so fediverse:// links would continue working for Fediverse users - they'd see a prompt to open the link with their home instance's web app (its web interface like the default web UI) or a dedicated web app they are already using like Voyager. What would need to change is just a minor thing: browsers would need to offer the default web UI of target instance as fallback: for example, even if you haven't visited any Fediverse site yet, the link fediverse://lemmy.example.com/post/1337 will show "lemmy.example.com web interface" in the "Open with..." option list, redirecting to https://lemmy.example.com/post/1337.