diff --git a/doc/conf.py b/doc/conf.py
index 17a9d22427414..60561431e7713 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -40,8 +40,7 @@
     'sphinx.ext.intersphinx',
     'sphinx.ext.imgconverter',
     'sphinx_gallery.gen_gallery',
-    'sphinx_issues',
-    'custom_autosummary_new_suffix'
+    'sphinx_issues'
 ]
 
 # this is needed for some reason...
@@ -456,16 +455,3 @@ def setup(app):
 warnings.filterwarnings("ignore", category=UserWarning,
                         message='Matplotlib is currently using agg, which is a'
                                 ' non-GUI backend, so cannot show the figure.')
-
-# Used by custom extension: `custom_autosummary_new_suffix` to change the
-# suffix of the following functions. This works around the issue with
-# `sklearn.cluster.dbscan` overlapping with `sklearn.cluster.DBSCAN`  on
-# case insensitive file systems.
-custom_autosummary_names_with_new_suffix = {
-    'sklearn.cluster.dbscan',
-    'sklearn.cluster.optics',
-    'sklearn.covariance.oas',
-    'sklearn.decomposition.fastica'
-}
-custom_autosummary_new_suffix = '-lowercase.rst'
-custom_autosummary_generated_dirname = os.path.join('modules', 'generated')
diff --git a/doc/sphinxext/custom_autosummary_new_suffix.py b/doc/sphinxext/custom_autosummary_new_suffix.py
deleted file mode 100644
index 5d51475b92a64..0000000000000
--- a/doc/sphinxext/custom_autosummary_new_suffix.py
+++ /dev/null
@@ -1,88 +0,0 @@
-"""Patches process_generate_options in sphinx.ext.autosummary to add change
-the suffix of items in configuration option
-custom_autosummary_names_with_new_suffix to
-custom_autosummary_new_suffix. For example:
-
-```python
-custom_autosummary_names_with_new_suffix = {
-    "sklearn.cluster.dbscan",
-    "sklearn.cluster.optics",
-    "sklearn.covariance.oas",
-    "sklearn.decomposition.fastica"
-}
-custom_autosummary_new_suffix = "-lowercase.rst"
-custom_autosummary_generated_dirname = os.path.join('modules', 'generated')
-
-This extension monkeypatches `os.path.join` used by
-https://github.com/sphinx-doc/sphinx/blob/7ffd6ccee8b0c6316159c4295e2f44f8c57b90d6/sphinx/ext/autosummary/generate.py#L168
-to return a filename with a new suffix.
-```
-"""
-import os
-import sphinx
-from contextlib import contextmanager
-
-from sphinx.ext.autosummary import get_rst_suffix
-from sphinx.ext.autosummary import process_generate_options
-
-
-@contextmanager
-def patch_os_path_join(generated_dirname, filename_map):
-    orig_os_path_join = os.path.join
-
-    def custom_os_path_join(a, *p):
-        path = orig_os_path_join(a, *p)
-        if len(p) != 1:
-            return path
-
-        dirname, name_with_suffix = os.path.split(path)
-        if not (dirname.endswith(generated_dirname) and
-                name_with_suffix in filename_map):
-            return path
-
-        return orig_os_path_join(dirname, filename_map[name_with_suffix])
-
-    os.path.join = custom_os_path_join
-    yield
-    os.path.join = orig_os_path_join
-
-
-def process_generate_options_custom_files(app):
-    orig_suffix = get_rst_suffix(app)
-    new_suffix = app.config.custom_autosummary_new_suffix
-    generated_dirname = app.config.custom_autosummary_generated_dirname
-    filename_map = {
-        name + orig_suffix: name + new_suffix
-        for name in app.config.custom_autosummary_names_with_new_suffix
-    }
-
-    with patch_os_path_join(generated_dirname, filename_map):
-        process_generate_options(app)
-
-
-def setup(app):
-    app.setup_extension('sphinx.ext.autosummary')
-    app.add_config_value(
-        'custom_autosummary_names_with_new_suffix', set(), None)
-    app.add_config_value(
-        'custom_autosummary_new_suffix', '-lowercase.rst', None)
-    app.add_config_value(
-        'custom_autosummary_generated_dirname', '', None)
-
-    if sphinx.version_info[0] <= 2:
-        raise ModuleNotFoundError("Please install Sphinx >= 3.0 in order "
-                                  "to build docs")
-
-    # Find listener id for process_generate_options added by
-    process_generate_options_id = None
-    builder_inited_listeners = app.events.listeners["builder-inited"]
-    for event_listener in builder_inited_listeners:
-        func = event_listener.handler
-        if func.__name__ == "process_generate_options":
-            process_generate_options_id = event_listener.id
-            break
-    assert process_generate_options_id is not None
-
-    # Override process_generate_options added by sphinx.ext.autosummary
-    app.disconnect(process_generate_options_id)
-    app.connect("builder-inited", process_generate_options_custom_files)