See https://github.com/embulk/embulk for Embulk's main repository. This is the dedicated repository for Embulk's Service Provider Interface (SPI), segregated from the main repository.
The Embulk SPI has had the two-digit versioning strategy since v0.11, such as v0.11, v1.0, v1.1, ..., which is independent from Embulk core's three-digit versioning strategy.