When redesigning the profiler and the toolbar, I put a constraint on allowing third-party bundles to support the old and the new design at the same time in their own panel (so that they don't have to drop support for Symfony <2.8 when updating to support the new design).

We need to document this feature to make bundle authors aware of it. The explanation about how it works is available in symfony/symfony#15160 (comment).
Note that the recommended usage of the variable should be documented, to ensure your bundle works fine when we bump the variable in the future (the comment I linked assumes a case where we bumped it several times in the future to explain how it would work).