Open source: Yes (BSD-3-Clause)
Repository: https://github.com/wangqr/Aegisub (build from autogenerated tarball doesn't work, because configure script gets version number from git). Latest version is 3.3.2.
Aegisub was deprecated from the repo some time ago, since upstream development is dead (last release was made in 2014, and last commit was in October 2019). I myself have offered Subtitle Composer as an alternative in T8803, but, unfortunately, SC still isn't as feature-rich as Aegisub - namely it completely misses support for SSA/ASS styling (even master branch doesn't have it, though there is an open issue) and translation workflow isn't as polished.
This fork incorporates all unreleased changes since 2014, continues development and tags new releases. I can confirm that latest 3.3.2 release can be compiled and used without any patches (as opposed to last 3.2.2 release of original Aegisub, for which we had to apply lots of patches just for compilation against latest libraries).
If this fork is accepted, I propose myself as maintainer.