@uroni, is there anything the community can do to help? I was wanting to port my server over to a Pi 4 board which requires Raspbian Buster which is based on Debian 10.
Apparently this libcurl issue impacted a lot of packages, but it was decided to move forward with the breaking change anyway: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=900511 and https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=858398
If I understand correctly, due to some “botched upgrades” in the past on an earlier version of Debian libcurl3 can’t be installed with libcurl4 (as mentioned by JML above). Debian 10’s changes are attempting to put everything right with libcurl but they can’t make 3 and 4 coinstallable. And libcurl3 won’t be supported on Debian 10 due to Debian dropping support for SSL 1.0 (which is a good move security wise).