diff --git a/readthedocs/doc_builder/exceptions.py b/readthedocs/doc_builder/exceptions.py
index 2ffa216bad4..abc7783522d 100644
--- a/readthedocs/doc_builder/exceptions.py
+++ b/readthedocs/doc_builder/exceptions.py
@@ -50,6 +50,7 @@ class BuildUserError(BuildBaseException):
     BUILD_COMMANDS_IN_BETA = "build:user:build-commands-config-key-in-beta"
     BUILD_TIME_OUT = "build:user:time-out"
     BUILD_EXCESSIVE_MEMORY = "build:user:excessive-memory"
+    VCS_DEPRECATED = "build:vcs:deprecated"
 
 
 class BuildMaxConcurrencyError(BuildUserError):
diff --git a/readthedocs/notifications/messages.py b/readthedocs/notifications/messages.py
index ed963a52831..b359fb15b59 100644
--- a/readthedocs/notifications/messages.py
+++ b/readthedocs/notifications/messages.py
@@ -188,6 +188,19 @@ def get_rendered_body(self):
         ),
         type=ERROR,
     ),
+    Message(
+        id=BuildUserError.VCS_DEPRECATED,
+        header=_("Build used a deprecated VCS is not supported: {{vcs}}."),
+        body=_(
+            textwrap.dedent(
+                """
+                {{vcs}} VCS is not supported anymore.
+                Read more about this in our blog post <a href="https://about.readthedocs.com/blog/2024/02/drop-support-for-subversion-mercurial-bazaar/">Dropping support for Subversion, Mercurial, and Bazaar</a>.
+            """
+            ).strip(),
+        ),
+        type=ERROR,
+    ),
     Message(
         id=BuildAppError.BUILD_DOCKER_UNKNOWN_ERROR,
         header=_("Build terminated due to unknown error."),
diff --git a/readthedocs/vcs_support/backends/bzr.py b/readthedocs/vcs_support/backends/bzr.py
index a065ed1b870..bbe63bd5422 100644
--- a/readthedocs/vcs_support/backends/bzr.py
+++ b/readthedocs/vcs_support/backends/bzr.py
@@ -3,13 +3,14 @@
 import csv
 import re
 from io import StringIO
+
 from django.conf import settings
 
 from readthedocs.projects.exceptions import RepositoryError
-from readthedocs.vcs_support.base import BaseVCS, VCSVersion
+from readthedocs.vcs_support.base import BaseVCS, Deprecated, VCSVersion
 
 
-class Backend(BaseVCS):
+class Backend(Deprecated, BaseVCS):
 
     """Bazaar VCS backend."""
 
diff --git a/readthedocs/vcs_support/backends/hg.py b/readthedocs/vcs_support/backends/hg.py
index fda4a4b7e27..6bf2e9cfc74 100644
--- a/readthedocs/vcs_support/backends/hg.py
+++ b/readthedocs/vcs_support/backends/hg.py
@@ -2,10 +2,10 @@
 from django.conf import settings
 
 from readthedocs.projects.exceptions import RepositoryError
-from readthedocs.vcs_support.base import BaseVCS, VCSVersion
+from readthedocs.vcs_support.base import BaseVCS, Deprecated, VCSVersion
 
 
-class Backend(BaseVCS):
+class Backend(Deprecated, BaseVCS):
 
     """Mercurial VCS backend."""
 
diff --git a/readthedocs/vcs_support/backends/svn.py b/readthedocs/vcs_support/backends/svn.py
index 83ed38c4757..dc220610e8d 100644
--- a/readthedocs/vcs_support/backends/svn.py
+++ b/readthedocs/vcs_support/backends/svn.py
@@ -6,10 +6,10 @@
 from django.conf import settings
 
 from readthedocs.projects.exceptions import RepositoryError
-from readthedocs.vcs_support.base import BaseVCS, VCSVersion
+from readthedocs.vcs_support.base import BaseVCS, Deprecated, VCSVersion
 
 
-class Backend(BaseVCS):
+class Backend(Deprecated, BaseVCS):
 
     """Subversion VCS backend."""
 
diff --git a/readthedocs/vcs_support/base.py b/readthedocs/vcs_support/base.py
index 678224303c2..5d79a9e7ca4 100644
--- a/readthedocs/vcs_support/base.py
+++ b/readthedocs/vcs_support/base.py
@@ -1,6 +1,8 @@
 """Base classes for VCS backends."""
+import datetime
 import os
 
+import pytz
 import structlog
 
 from readthedocs.core.utils.filesystem import safe_rmtree
@@ -33,6 +35,46 @@ def __repr__(self):
         )
 
 
+class Deprecated:
+    def __init__(self, *args, **kwargs):
+        tzinfo = pytz.timezone("America/Los_Angeles")
+        now = datetime.datetime.now(tz=tzinfo)
+
+        # Brownout dates as published in https://about.readthedocs.com/blog/2024/02/drop-support-for-subversion-mercurial-bazaar/
+        # fmt: off
+        disabled = any([
+            # 12 hours browndate
+            datetime.datetime(2024, 4, 1, 0, 0, 0, tzinfo=tzinfo) < now < datetime.datetime(2024, 4, 1, 12, 0, 0, tzinfo=tzinfo),
+            # 24 hours browndate
+            datetime.datetime(2024, 5, 6, 0, 0, 0, tzinfo=tzinfo) < now < datetime.datetime(2024, 5, 7, 0, 0, 0, tzinfo=tzinfo),
+            # 48 hours browndate
+            datetime.datetime(2024, 5, 20, 0, 0, 0, tzinfo=tzinfo) < now < datetime.datetime(2024, 5, 22, 0, 0, 0, tzinfo=tzinfo),
+            # Deprecated after June 3
+            datetime.datetime(2024, 6, 3, 0, 0, 0, tzinfo=tzinfo) < now,
+        ])
+        # fmt: on
+
+        if disabled:
+            from .backends import bzr, hg, svn
+
+            vcs = None
+            if isinstance(self, bzr.Backend):
+                vcs = "Bazaar"
+            elif isinstance(self, svn.Backend):
+                vcs = "Subversion"
+            elif isinstance(self, hg.Backend):
+                vcs = "Mercurial"
+
+            raise BuildUserError(
+                message_id=BuildUserError.VCS_DEPRECATED,
+                format_values={
+                    "vcs": vcs,
+                },
+            )
+
+        super().__init__(*args, **kwargs)
+
+
 class BaseVCS:
 
     """