According to:
https://aegisub.org/blog/aegisub-3.4.0-released/#extra-notes-for-distros

Aegisub can’t be built with LTO.

Is there something that breaks under LTO currently? I just compiled Aegisub with no modifications, FFMS2 and avisynth disabled, and Clang+ThinLTO on OpenBSD, and it seems to work.

This is important because some systems already moved to LTOing everything: https://fedoraproject.org/wiki/LTOByDefault
http://www.ucw.cz/~hubicka/slides/opensuse2018-e.pdf

Others will probably follow. And it's always possible that e.g. a Gentoo or NetBSD user is compiling his packages with it because those have 1st class support for source-based package management and the user can change compile flags.