Luabins is not directly used anywhere by Aegisub itself, as far as I can tell it was only added as a convenience library for automation scripts, especially for serializing tables to save as line extradata. However, I'm not aware of any script that actually uses it. (AegisubMotion serializes extradata using the json module shipped with DependencyControl.)

Luabins is a vendored dependency in Aegisub and upstream is no longer maintained, so with all this in mind it may wise to phase out luabins over time, e.g. by making it print a deprecation warning for a while and then eventually removing it fully.

If someone does know of a script that uses luabins, please let me know here.