VariableFrontend treats all Backends like TaggableBackendInterface-Backends
The SimpleFileBackend does not implement the TaggableBackendInterface but the VariableFrontend treats all Backends like TaggableBackendInterface-Backends.
Thus, the method getByTag fails with an exception when used with a SimpleFileBackend.
There is clearly no simple solution to it but it's a bummer when configuring a cache like this and trying to use tags. One approach to fix this is to remove the tag-related methods from the FrontendInterface, introduce a TaggableFrontendInterface and only let custom cache frontends implement the optional TaggableFrontendInterface.