6.8, and bcrypt for new password hashes

WordPress changes its password hashing in April, and the migration completes only as fast as users log in.

the mechanism: new hashes use bcrypt; an existing
phpass hash is verified with the old algorithm and
rehashed on the next successful login.

four weeks after 6.8, on 8,104 users:

  bcrypt      1,208   (15%)
  phpass      6,896   (85%)

and the long tail is users who log in annually, which
means the old algorithm has to stay supported for
years rather than months.

A rehash-on-login migration has no completion date, which is the correct design and means the compatibility code is permanent in practice. The measurable cost is per-login: bcrypt at the default cost is about 60ms against phpass at about 25ms, which is invisible to a user and doubles the CPU of a login flood.