The age and obscurity of the library is irrelevant - you could always include libraries bundled with the app, if they didn't exist in system repos. For example, in deb packages, you could include it in the data.tar portion of the package (see https://en.m.wikipedia.org/wiki/Deb_(file_format)).

Libraries with version names baked in are one solution to the dependency hell problem, but that requires support from the language/framework/tooling to build the application, and/or the OS (or things get hacky and messy quickly).

If you read that dependency hell page, you'll see another solution is portable apps, which specifically mentions Appimage, Flatpak, and Snap.

Additionally, if you read the Debian docs on How to Cope with Conflicting Requirements, the first solution they give is to "Install such programs using corresponding sandboxed upstream binary packages," such as "Flatpak, Snap, or AppImage packages."

Bin the consumer environment? It is nice and good practice but it is nowhere near as important as it used to be.

This is incorrect. The target audience for Flatpak is desktop users: https://docs.flatpak.org/en/latest/introduction.html#target-audience. Flatpaks are explicitly for consumer, graphical applications.