That sounds like more something to blame the Debian family over than the whole ecosystem. I might be running "bleeding edge" on Arch but I can still depend on and use libcurl3 with the libcurl-compat package.
I maintain about two dozen software packages in the AUR, including some really old stuff like the Heretic 2 Linux release from 1999 and RBDoom 3 BFG which has a boatload of dependencies. Breakages are extremely rare for the average package even with the rolling release and generally any breaking change in a common library will see the legacy version hang around since stuff will still depend on it.
I don't think it's Debian, because Debian tends to have compatibility packages for old versions too. For libcurl there's libcurl3. It depends on the package, of course.
I don't have a Debian machine to test it on but a casual search finds qt1-3 in the AUR on Arch. Of course building the whole GUI suite would be a bit annoying but its better than nothing if you have some really old software that depends on it.
I maintain about two dozen software packages in the AUR, including some really old stuff like the Heretic 2 Linux release from 1999 and RBDoom 3 BFG which has a boatload of dependencies. Breakages are extremely rare for the average package even with the rolling release and generally any breaking change in a common library will see the legacy version hang around since stuff will still depend on it.