Is it possible to enable
Extended Support for Accessing Lightning Experience Using Microsoft Internet Explorer 11 for Lightning Communities only?
Such that the rest of Salesforce Lightning does not have it enabled.
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Apparently, normal IE 11 support is available without Extended Support:
Are Salesforce Classic, Console, or Communities affected by this change?
There is no impact to Salesforce Classic, as IE11 will remain a supported browser. This includes the use of Lightning components in Visualforce pages running in Salesforce Classic.
Additionally, there is no impact to Communities (Visualforce and Lightning template-based), Salesforce Console for Salesforce Classic, Lightning for Outlook, or standalone Lightning apps (e.g. myapp.app).
I can't test this, but it seems that IE 11 support could be disabled without affecting your communities.