The protocol extensions in this folder are considered implementation details of Qt. I.e. they may removed, renamed or changed without warning.
However, starting with Qt 5.4, we promise not to break backwards compatibility without renaming (or removing) the protocol. I.e., if your client sees a global from one of these extensions, it can safely bind to it: the existing events and requests will always take the same number of arguments, regardless of compositor version.
This is important also within a Qt-only scope if there are multiple versions of Qt on the system. Consider for instance an application statically linked to Qt (such as Qt Creator) running against a Qt compositor installed by the distro). In such cases we don't want the compositor and client to disagree on the protocol definition.
Protocol extensions in this folder should be versioned (e.g. zqt_key_v1). If it is necessary to break protocol compatibility, they will be renamed by incrementing the version number. For legacy reasons, there are also unversioned protocols in this folder. Those protocols should be renamed to be versioned if compatibility is broken.