Remove support for miniTocMaxHeadingLevel (#34758)

lib/frontmatter.js

@@ -42,7 +42,9 @@ export const schema = {
     showMiniToc: {
       type: 'boolean',
     },
+    // This frontmatter property is deprecated. Despite what `miniTocMaxHeadingLevel` says, the max level of mini TOC is *always* 2. See github/docs-engineering#2701.
     miniTocMaxHeadingLevel: {
+      deprecated: true,
       type: 'number',
       default: 2,
       minimum: 2,
@@ -237,6 +239,11 @@ export const schema = {
   },
 }
 
+// returns a list of deprecated properties
+export const deprecatedProperties = Object.keys(schema.properties).filter((prop) => {
+  return schema.properties[prop].deprecated
+})
+
 const featureVersionsProp = {
   feature: {
     type: ['string', 'array'],
@@ -266,7 +273,7 @@ schema.properties.versions = {
   }, featureVersionsProp),
 }
 
-function frontmatter(markdown, opts = {}) {
+export function frontmatter(markdown, opts = {}) {
   const defaults = {
     schema,
     validateKeyNames: true,