Computer Configuration > Administrative Templates > Microsoft Edge - Default Settings (users can override)
Both
HKLM\Software\Policies\Microsoft\Edge\Recommended
Set this policy to show the Reload in Internet Explorer mode button in the toolbar. Users can hide the button in the toolbar through edge://settings/appearance. The button will only be shown on the toolbar when the 'InternetExplorerIntegrationReloadInIEModeAllowed' (Allow unconfigured sites to be reloaded in Internet Explorer mode) policy is enabled or if the user has chosen to enable "Allow sites to be reloaded in Internet Explorer mode". If you enable this policy, the Reload in Internet mode button is pinned to the toolbar. If you disable or don't configure this policy, the Reload in Internet Explorer mode button isn't shown in the toolbar by default. Users can toggle the Show Internet Explorer mode button in edge://settings/appearance.