diff --git a/Changelog.md b/Changelog.md
index 1beecb663..8a73168f6 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -11,6 +11,10 @@ Bug Fixes:
 * Support keyword argument semantics when constraining argument expectations using
   `with` on Ruby 3.0+ (Yusuke Endoh, #1394)
 
+Deprecations:
+
+* Add RSpec 4 deprecation warnings. (Phil Pirozhkov, #????)
+
 ### 3.10.2 / 2021-01-27
 [Full Changelog](http://github.com/rspec/rspec-mocks/compare/v3.10.1...v3.10.2)
 
diff --git a/lib/rspec/mocks/configuration.rb b/lib/rspec/mocks/configuration.rb
index 5962215f8..f22d833cf 100644
--- a/lib/rspec/mocks/configuration.rb
+++ b/lib/rspec/mocks/configuration.rb
@@ -80,6 +80,9 @@ def add_stub_and_should_receive_to(*modules)
       #
       def syntax=(*values)
         syntaxes = values.flatten
+        RSpec.deprecate('Mocks syntax configuration',
+                        :replacement => 'the default `expect` syntax',
+                        :call_site => nil)
         if syntaxes.include?(:expect)
           Syntax.enable_expect
         else
@@ -87,6 +90,9 @@ def syntax=(*values)
         end
 
         if syntaxes.include?(:should)
+          RSpec.deprecate('`:should` Mocks syntax',
+                          :replacement => 'the default `expect` syntax',
+                          :call_site => nil)
           Syntax.enable_should
         else
           Syntax.disable_should
diff --git a/lib/rspec/mocks/example_methods.rb b/lib/rspec/mocks/example_methods.rb
index 5531b28bf..26171ef7f 100644
--- a/lib/rspec/mocks/example_methods.rb
+++ b/lib/rspec/mocks/example_methods.rb
@@ -199,6 +199,9 @@ def class_spy(*args)
       # early on.
       # @deprecated Use {RSpec::Mocks::Configuration#allow_message_expectations_on_nil} instead.
       def allow_message_expectations_on_nil
+        # require 'pry'; binding.pry
+        RSpec.deprecate("`allow_message_expectations_on_nil` example method",
+                        :replacement => "`allow_message_expectations_on_nil` configuration option")
         RSpec::Mocks.space.proxy_for(nil).warn_about_expectations = false
       end
 
diff --git a/spec/rspec/mocks/configuration_spec.rb b/spec/rspec/mocks/configuration_spec.rb
index dd1a00f7a..8d72aec26 100644
--- a/spec/rspec/mocks/configuration_spec.rb
+++ b/spec/rspec/mocks/configuration_spec.rb
@@ -68,6 +68,11 @@ def sandboxed
             expect(::RSpec::Mocks::ExampleMethods).not_to receive(:method_added)
             configure_syntax :expect
           end
+
+          it 'emits a deprecation warning' do
+            expect_deprecation_without_call_site(/Mocks syntax configuration/)
+            configure_syntax :expect
+          end
         end
 
         context 'when configured to :should' do
@@ -89,14 +94,15 @@ def sandboxed
             expect(configured_syntax).to eq([:should])
           end
 
-          it "does not warn about the should syntax" do
-            RSpec.should_not_receive(:deprecate)
-            Object.new.should_not_receive(:bees)
-          end
-
           it 'is a no-op when configured a second time' do
             Syntax.default_should_syntax_host.should_not_receive(:method_added)
             ::RSpec::Mocks::ExampleMethods.should_not_receive(:method_undefined)
+          end
+
+          it 'emits two deprecation warnings' do
+            configure_syntax :expect
+            expect_deprecation_without_call_site(/`:should` Mocks syntax/)
+            expect_deprecation_without_call_site(/Mocks syntax configuration/)
             configure_syntax :should
           end
         end
@@ -124,6 +130,12 @@ def sandboxed
             expect(RSpec).not_to receive(:deprecate)
             expect(Object.new).not_to receive(:bees)
           end
+
+          it 'emits two deprecation warnings' do
+            expect_deprecation_without_call_site(/`:should` Mocks syntax/)
+            expect_deprecation_without_call_site(/Mocks syntax configuration/)
+            configure_syntax [:should, :expect]
+          end
         end
       end
 
diff --git a/spec/rspec/mocks/example_methods_spec.rb b/spec/rspec/mocks/example_methods_spec.rb
index 36bb71623..846a32605 100644
--- a/spec/rspec/mocks/example_methods_spec.rb
+++ b/spec/rspec/mocks/example_methods_spec.rb
@@ -33,6 +33,17 @@ def test_extend_on_new_object(*to_extend, &block)
           expect(dbl.foo).to eq(1)
         end
       end
+
+      describe '#allow_message_expectations_on_nil' do
+        it "emits a deprecation warning on use" do
+          expect_deprecation_with_call_site(__FILE__, __LINE__ + 3, /allow_message_expectations_on_nil/)
+          RSpec.describe do
+            it do
+              allow_message_expectations_on_nil
+            end
+          end.run
+        end
+      end
     end
   end
 end