The present implementation uses code (MCRSolrProxyUtils#getQueryHandlerList()) to determine which solr query handlers are available and which of them are restricted (by the property MCR.Module-solr.Proxy.ClassFilter). This is a blacklist approach.
The current code does not work with solr 7. Additionally to code is complex and error prone, there are query handers we simply missed. E.g. (/update/extract) which is allowed by the default configuration.
The new implementation will use a simple configurable (by property) whitelist. This will reduce code complexity and will probably work for all future versions of solr.