InstantSearch.js - How can I hide a filter with only one facet value?

You can use a panel widget to access the results, and use the hidden function to hide a refinement based on how many facet values it has:

instantsearch.widgets.panel({
  hidden(options) {
    const facetValues = options.results.getFacetValues('brand');
    return Array.isArray(facetValues) ? facetValues.length <= 1 : false;
  },
})(instantsearch.widgets.refinementList)({
  container: '#brand-list',
  attribute: 'brand',
})

Applies to InstantSearch v3 and later.