You clearly don't understand the purpose of LTS distributions of system
software.
After 55+ years in the computer industry, that is an insult. I have worked on everything from embedded systems to supercomputers and I have managed large groups both locally and internationally. I think you don't understand software development of a moving target system like duplicity.
The moving target is over 30 downstream cloud providers that constantly change their API, release it to PyPi, and do not provide any backwards compatibility. When that happens, the backend changes, and the version changes, either at the minor or patch level depending on severity. The change is put out via git and upstream maintainers are free to create whatever patches they need to update the 1 or 2 files changed, plus update the Python module provided by the cloud provider. Now that semantic version change has locked duplicity out of its current distro with more to come from my downstream. You see, duplicity sits midstream of all that; distro maintainers are my upstream, and the cloud providers are my downstream. Without 30+ downstream providers, duplicity would be like other software, only changing when major improvements come along.
Thus artificially limiting a new software version to not run on LTS
distributions is as silly as limiting it to only run on the latest CPU,
because ultimately the computer rarely exists solely for the pleasure
of one software writer.
The artificial limit is in your imagination. All of the distros are more than 2 years behind duplicity's current version anyway. There's your limit.
What duplicity needs is more latitude in what I see as a locked-in LTS system that does not account for midstream providers.
In all honesty, I don't have a solution to your complaint. I am fighting to get duplicity modernized while LTS is fighting for stagnation. I think there may be a middle ground, but I don't know where.
I do know that in all the distros you listed, duplicity will quite happily run in a snap since it isolates the environment from the stagnant system. It will also run in a venv isolated from the system, so I'm not too worried about clobbering the LTS systems, I'm just trying to get duplicity up-to-date technically. duplicity has a large technical debt after Py2.
...Ken