Before Groovy 2.0.0 we followed a version scheme where we have X.Y.Z, where X.Y is the major version, and Z the minor version. Bugfix versions where not really done, you had to upgrade to the next minor version for that. Since Groovy 1.0 we incremented only the Y for a new major version. The increment of X we wanted to leave for a very big breaking change, like a new MOP. The last major version in these scheme is 1.8(.0), 1.8.1 is the first minor and bugfix version. The major versions in the past using this scheme are: 1.8, 1.7, 1.6, 1.5, 1.0. Each of them having around 10 minor/bugfix versions.
Official Major Version:
The official major version is the current major version that should/can be used by the developers if they are not bound to a specific major version.
Maintenance Release Branch:
Here we indicate a former major version's bugfix release.
How long is a major version maintained?
That depends on the users. Let's say we have X in maintenance and Y is the official major version, then if a new major version Z is released, Y goes into maintenance. Usually we make one or two more bugfix releases for X and then we discontinue it, unless there are strong requests to have certain things fixed for users that can absolutely not upgrade.