Here's all the circumstances surrounding it:

• It was composed by Ridiculon for use in Super Meat Boy Forever.
• Tommy Refenes (programmer and co-designer of Super Meat Boy and SMBF) first uploaded this track under the filename QuestionMark.ogg in the #sneak-peek channel of Team Meat's Discord server at 6 : 34 AM (EST) Febuary 1st, 2019. [March 19th update] It has not yet been revealed what the track is actually named, but we know that it will be used in the light world for a desert-themed chapter.
• No higher-quality version of this track is publicly available as of yet; however, the version you hear by mousing over the thumbnail is somewhat harshly compressed to fit The Modding of Isaac's 1mb size limit for audio previews. The version actually used in the mod is identical to what Tommy uploaded.
• Maybe it wasn't a "leak" so much as a developer-endorsed preview, but considering how rare it is for Ridiculon to show off previews, and how they weren't the ones who uploaded it...

This mod was made for Afterbirth+, but should work in Rebirth and Afterbirth since there's no lua scripting involved.
QuestionMark.ogg replaces both regular boss themes (Crusade and Cerebrum Dispersio), has no intro, and uses Cerebrum Dispersio's outro.
It loops cleanly and has not been edited in any way (except for the preview version, read above).
This is not compatible with other mods that replace any aspect of the two regular boss themes or redefine said boss themes in music.xml.

2021 edit: SMBF released about 2 years after this mod (December 23rd, 2020). The track used here turns out to be a time-shifted version of Tenuous (Tetanusville light). Or, the in-game version, since the OST version is like a remix for some reason.