Computer Configuration > Administrative Templates > Microsoft Edge > Immersive Reader settings
Both
HKLM\Software\Policies\Microsoft\Edge
OBSOLETE: This policy is obsolete and doesn't work after Microsoft Edge 125. This policy is obsoleted because Grammar Tools is deprecated from Edge. This policy won't work in Microsoft Edge version 126. Enables the Grammar Tools feature within Immersive Reader in Microsoft Edge. This helps improve reading comprehension by splitting words into syllables and highlighting nouns, verbs, adverbs, and adjectives. If you enable this policy or don't configure it, the Grammar Tools option shows up within Immersive Reader. If you disable this policy, users can't access the Grammar Tools feature within Immersive Reader.