The 6.15.1 release is a patch to ensure that a layer gets rendered when its source has completed loading. See the changelog for 6.15.0 for new features and fixes since 6.14.

List of all changes:

• Recover from incorrect rebase after removal of string enums (in #13835 by @ahocevar)

Full Changelog: https://github.com/openlayers/openlayers/compare/v6.15.0...v6.15.1