If a binary library ABI is backwards-incompatible, they change the package name. I was just guessing at the reason epoch is avoided and that <6 is probably an awful example.
So now I actually bothered to look it up, and it turns out the actual reason is that the epoch changes what version is considered "greater", but it's not part of the .deb filename, so you still can't reuse version numbers used in the past. If you release 5.0, then 5.1, then you want to rollback and release 1:5.0, it's going to break things in the Debian archives. https://www.debian.org/doc/debian-policy/ch-binary.html#uniq...
Additionally, once you add an epoch you're stuck with it forever, while if you use 5.1+really5.0, you can get rid of the kludge when 5.2 is out. https://www.debian.org/doc/debian-policy/ch-controlfields.ht...
