Juicebox officially supports "Internet Explorer 7+, Firefox 3.6+, Chrome, Safari and Opera." (as noted in the System Requirements).

I notice that there is one line of code in the 'theme.css' file which is noted to be "For IE 5.5 - 7" so it'll be there for IE7 support.
There should no no code specifically for versions of IE below IE7 (and very little code specifically for versions below IE11).

Maybe it is time to leave IE7 behind (it certainly doesn't have a large market share any more) but removing the IE7-specific code would not reduce the overall size of the Juicebox core files by much.
The developers have likely opted to retain IE7 support (at least for the time being) as removing support would save only a KB or two.

According to this report, IE8 still has a 6.21% market share (May 2016) so it might not yet be time to remove support for IE8.

If you like, you could post your suggestion in the Feature Requests thread.
This keeps all the ideas together and ensures that they are not overlooked by the developers.
Thank you.