Is it that complicated to use a slightly different build process in order to comply with distribution (at least LTS releases) dependancies?
Branch Ubuntu/Bionic built against libs included there (incl. libcurl4)
Branch Ubuntu/Trusty built against libs included there (incl. libcurl3)
And reflecting that in the hierarchical repository as most other PPAs do well like GIMP or Darktable
http://ppa.launchpad.net/otto-kesselgulasch/gimp/ubuntu/dists/
http://ppa.launchpad.net/pmjdebruijn/darktable-unstable/ubuntu/
Yes that's a bit more effort, but would solve all these issues