BIND 9.15 is an unstable development branch. When its development is complete, it will be renamed to BIND 9.16, which will be a stable branch. The end of life date for BIND 9.16 has not yet been determined. For those needing long term support, the current Extended Support Version (ESV) is BIND 9.11, which will be supported until at least December 2021. See https://kb.isc.org/docs/aa-00896 for details of ISC's software support policy.