For the record: a Sigil ppa would have been a possibility quite a while ago. And it's a definite possibility now. But there for a while, the dependencies of Sigil simply couldn't be
met by LTS systems.
So while a Sigil ppa could be (and often was) attempted by interested third-parties, one was in no better shape when you STILL had to go way outside the LTS (and even newer non-LTS) box for sufficient versions of Qt5 and Python3 for Sigil to work. You'd have needed three ppas: one providing bleeding edge Qt5; one providing a bleeding edge Python3 and one providing Sigil (and a prayer that binaries could be provided that didn't exceed the LTS's glibc limitations due to Sigil's C++11 compiler requirements). I think 14.04
finally got to Python 3.4 just relatively recently, but I think it was still way shy of Sigil's Qt5.4.0 minimum when 16.04 came out.
Now that LTS has caught up to Sigil's dependencies, there's really no need for a ppa anymore--though there's definitely one out there. It's maintainer has been patiently keeping it up to date and ready for whenever kicking it up to Ubuntu's repos finally made sense. It just wouldn't have really helped you before 16.04.
And I'm a fairly recent convert to Arch Linux, as well. The previous "upgrade" from an earlier LTS cured me from ever wanting to go down that road again. My 14.04-based Linux Mint 17 machine will likely rot.