diff --git a/wagtail/core/rich_text/__init__.py b/wagtail/core/rich_text/__init__.py
index 2066ff2d0..f44971b7d 100644
--- a/wagtail/core/rich_text/__init__.py
+++ b/wagtail/core/rich_text/__init__.py
@@ -67,7 +67,10 @@ class EntityHandler:
     """
     @staticmethod
     def get_model():
-        return NotImplementedError
+        """
+        If supported, returns the type of model able to be handled by this handler, e.g. Page.
+        """
+        raise NotImplementedError
 
     @classmethod
     def get_instance(cls, attrs: dict) -> Model:
diff --git a/wagtail/core/rich_text/feature_registry.py b/wagtail/core/rich_text/feature_registry.py
index 9bef84be0..2319ceff8 100644
--- a/wagtail/core/rich_text/feature_registry.py
+++ b/wagtail/core/rich_text/feature_registry.py
@@ -1,4 +1,7 @@
+from warnings import warn
+
 from wagtail.core import hooks
+from wagtail.utils.deprecation import RemovedInWagtail27Warning
 
 
 class FeatureRegistry:
@@ -68,16 +71,40 @@ class FeatureRegistry:
         except KeyError:
             return None
 
-    def register_link_type(self, handler):
-        self.link_types[handler.identifier] = handler
+    def register_link_type(self, identifier_or_handler_obj, handler_fn=None):
+        if handler_fn is not None:
+            # invoked as register_link_type(identifier, handler_function) - deprecated
+            identifier = identifier_or_handler_obj
+            warn(
+                'FeatureRegistry.register_link_type(link_type, handler_function) is deprecated. '
+                'Use FeatureRegistry.register_link_type(handler_object) instead',
+                category=RemovedInWagtail27Warning
+            )
+            self.link_types[identifier] = self.function_as_entity_handler(identifier, handler_fn)
+        else:
+            # invoked as register_link_type(handler_object)
+            handler = identifier_or_handler_obj
+            self.link_types[handler.identifier] = handler
 
     def get_link_types(self):
         if not self.has_scanned_for_features:
             self._scan_for_features()
         return self.link_types
 
-    def register_embed_type(self, handler):
-        self.embed_types[handler.identifier] = handler
+    def register_embed_type(self, identifier_or_handler_obj, handler_fn=None):
+        if handler_fn is not None:
+            # invoked as register_embed_type(identifier, handler_function) - deprecated
+            identifier = identifier_or_handler_obj
+            warn(
+                'FeatureRegistry.register_embed_type(link_type, handler_function) is deprecated. '
+                'Use FeatureRegistry.register_embed_type(handler_object) instead',
+                category=RemovedInWagtail27Warning
+            )
+            self.embed_types[identifier] = self.function_as_entity_handler(identifier, handler_fn)
+        else:
+            # invoked as register_embed_type(handler_object)
+            handler = identifier_or_handler_obj
+            self.embed_types[handler.identifier] = handler
 
     def get_embed_types(self):
         if not self.has_scanned_for_features:
@@ -96,3 +123,11 @@ class FeatureRegistry:
             return self.converter_rules_by_converter[converter_name][feature_name]
         except KeyError:
             return None
+
+    @staticmethod
+    def function_as_entity_handler(identifier, fn):
+        """Supports legacy registering of entity handlers as functions."""
+        return type('EntityHandlerRegisteredAsFunction', (object,), {
+            'identifier': identifier,
+            'expand_db_attributes': staticmethod(fn),
+        })
diff --git a/wagtail/core/tests/test_rich_text.py b/wagtail/core/tests/test_rich_text.py
index 95b1547d8..2d62e2677 100644
--- a/wagtail/core/tests/test_rich_text.py
+++ b/wagtail/core/tests/test_rich_text.py
@@ -1,5 +1,6 @@
 from unittest.mock import patch
 
+from django.contrib.auth import get_user_model
 from django.test import TestCase
 
 from wagtail.core.rich_text import RichText, expand_db_html
@@ -95,6 +96,36 @@ class TestFeatureRegistry(TestCase):
             features.get_editor_plugin('hallo', 'made_up_feature')
         )
 
+    def test_legacy_register_link_type(self):
+        User = get_user_model()
+        User.objects.create(username='wagtail', email='hello@wagtail.io')
+
+        def user_expand_db_attributes(attrs):
+            user = User.objects.get(username=attrs['username'])
+            return '<a href="mailto:%s">' % user.email
+
+        features = FeatureRegistry()
+        features.register_link_type('user', user_expand_db_attributes)
+
+        handler = features.get_link_types()['user']
+        self.assertEqual(
+            handler.expand_db_attributes({'username': 'wagtail'}),
+            '<a href="mailto:hello@wagtail.io">'
+        )
+
+    def test_legacy_register_embed_type(self):
+        def embed_expand_db_attributes(attrs):
+            return '<div>embedded content: %s</div>' % attrs['content']
+
+        features = FeatureRegistry()
+        features.register_embed_type('mock_embed', embed_expand_db_attributes)
+
+        handler = features.get_embed_types()['mock_embed']
+        self.assertEqual(
+            handler.expand_db_attributes({'content': 'foo'}),
+            '<div>embedded content: foo</div>'
+        )
+
 
 class TestLinkRewriterTagReplacing(TestCase):
     def test_should_follow_default_behaviour(self):