If someone uses a plain 5.3.9 bundle (i.e. not his own webapp) and replaces WEB-INF/libs with those from a plain 5.4 bundle, we get a few ClassNotFoundExceptions. Although they will be harmless if one doesn't use STK.

info.magnolia.module.templatingkit.dam.STKAssetRenderer
info.magnolia.module.templatingkit.imaging.generation.STKImageGenerator
info.magnolia.module.devicedetection.filter.DeviceDetectionFilter
info.magnolia.module.devicedetection.DeviceChannelResolver

Obviously, STK is not bundled anymore, and a few other modules may not come transitively anymore (e.g. device-detection).

That said, I'm not sure we do anything about it—at least filing it for reference, who knows...