It seems that the Volto Search Block comes with facets such as 'review state' and 'type'. How do I go about adding additional facets? E.g. filtering by tag.
It's due to this function here: volto/schema.js at 4a760bde9bc2ea8050d7a16f2b518e244d415536 · plone/volto · GitHub There's some comments there mentioning the need for improvements.
I've tested, and if you remove that function the Subject (Tags) index actually shows up as an available facet (and works in the search block view mode). You could try to write a schemaEnhancer function for the search block that removes that property (or shadow customize the schema.js module), to remove the function.