On https://documentation.magnolia-cms.com/display/DOCS/Request+processing+and+filters#Requestprocessingandfilters-Extensionpoints, the table does hint that AbstractMgnlFilter can be extended but the requirement for such extension is obscure. Something like "custom filters must extend AbstractMgnlFilter" or "custom filters must implement MgnlFilter" in the document would be much clearer to developers.

I basically spent quite some time trying and couldn't figure out why the configuration workspace wouldn't successfully add my custom filter into Magnolia's filter chain. So it turned out that they must implement MgnlFilter. This is the reason for raising an improvement on the document.