Deprecate component eventManager

text/0000-deprecate-custom-event-manager.md

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+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: (leave this empty)
+- Ember Issue: (leave this empty)
+
+# Summary
+
+Support for component `eventManger`s is a seldom used feature and should
+be deprecated.
+
+# Motivation
+
+We should strive to simplify the Ember API and source code where possible. As
+the custom `eventManager` feature is rarely used in apps, we should deprecate
+it.
+
+# Detailed design
+
+We'll introduce a deprecation warning which will be displayed when a component
+defines an `eventManager` property or when `canDispatchToEventManager` is set to
+true on `EventDispatcher`. The warning will have a target version of `3.0`.
+
+If required, we can create an addon which extends the `EventDispatcher` allowing
+for opt-in custom `eventManager`s in Ember apps.
+
+# How We Teach This
+
+As this is a seldom used feature, we can simply note the deprecation in a
+future release blog post.
+
+# Drawbacks
+
+This adds a little more churn for apps that rely on this feature.
+
+# Alternatives
+
+This feature was [recently made pay-as-you-go](https://github.com/emberjs/ember.js/pull/14756),
+so the immediate performance concerns have been addressed. We could decide to
+leave this in the framework as an opt-in feature.