Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
If you enable or don't configure this policy, the user can use spellcheck. If you disable this policy, the user can't use spellcheck and the 'SpellcheckLanguage' (Enable specific spellcheck languages) and 'SpellcheckLanguageBlocklist' (Force disable spellcheck languages) policies are also disabled.