master is a branch for development on the repo. It should always be kept up-to-date like official, but here you can change and tweak things. This is the default branch and all Pull Requests go to this one.

official should always be kept up-to-date with the downloadable version from the homepage. Except this no things should be modified here.

latestgithub is protected and should not be changed. It shows the state before Threema added the notice to their repo that it is not maintained anymore.

Of latestgithub only one release was created. As the branch is not updated this will be the only release.

On master there are only minor differences to official, e.g. the doc folder is removed. All changes are committed to master and send to Threema. If Threema releases a new version on the Gateway website, this version is also released on GitHub.