1.5.0-beta.0 - ABI-compatible with 1.3.0
Pre-release
The 1.5.0 release will backport all the functionality and bugfixes of the 2.0.0 beta, but with a backwards compatible API/ABI* with a caveat, see below.
The intention is for the 1.5.0 release to be the dependency for MyPaint 2.0.0 and probably the next few MyPaint releases, reserving the 2.x versions of libmypaint for more fundamental changes to the API. This also means that the 2.0.0 release will be in beta for some time, or possibly even revert to alpha to reflect the stability of the API.
ABI compatibility
The 1.4.0 release broke ABI compatibility with 1.3.0 without setting the abi version fields accordingly. This wasn't noticed until last month. The issue is an enum reordering which will not result in crashes, but buggy/incorrect results if using 1.4.0 with an application compiled using the 1.3.0 headers.
Right now, the 1.5.0 prerelease is ABI compatible with 1.3.0, but if the consensus of package maintainers is that it really should be ABI compatible with 1.4.0 instead (if you think it matters at all), do voice your opinion in an issue.
It should be noted that libmypaint, for better or worse, has always used the libtool -release tag, in addition to -version-info.