Boost eligibility is covered in our distribution guidelines over here: https://help.medium.com/hc/en-us/articles/360006362473-Medium-s-quality-standards-how-we-review-stories-for-distribution

While a few topics like erotica and "meta" (stories about making money on Medium) are not eligible for Boost, broadly speaking, most topics are Boostable including poetry, fiction, humor, history, photography, etc. The focus is on quality, not topic... And we love niche topics!