After five years of 0.x, 1.0 in November is a commitment that a level will not start reporting new errors in a patch release.
$ composer require --dev phpstan/phpstan:^1.0
$ vendor/bin/phpstan analyse
# what 1.0 changes for a project already on 0.12:
# very little in findings — the levels are the same
# a lot in planning — an update is now safe to apply
# without a day set aside for new errors
# minor versions may still ADD levels. 9 arrives later.The practical difference is that the tool can now be updated on the same schedule as everything else, rather than treated as a change that needs its own branch. New rules land in new levels or behind configuration, which is the promise. It also means the baseline is stable — a 0.12 update could invalidate a baseline by changing an error message, which was the most annoying failure mode of the previous five years.