If you do not beatmap the last 20% of your beatmapset's audio file, it must be cut. The intro time is not included. This does not apply if more than 20% of the outro is occupied by a storyboard/video.
20% is arbitrarily defined, and there's no obvious intention stated along with this rule. the intro is also arbitrarily excluded
two routes for resolving this one:
1. remove it because it's not consistent with (the lack of) other requirements about excess content. it's common to waste multiple megabytes on shitty/misinformed JPEG encodes, lossless formats for lossy content, poor tuning of AVC encoders, etc., yet audio is required to be shortened for possibly much less of a benefit in beatmap size
2. define the "20%" better and make the reasoning of this rule more apparent (those probably go hand-in-hand). then probably open discussions about similar excess content problems