Apparently, just redirecting to the directory stopped working, but linking to index.html works fine.
@@ -3,7 +3,7 @@
<head>
<title>Redirecting to https://holzhaus.github.io/sphinx-multiversion/master/</title>
<meta charset="utf-8">
- <meta http-equiv="refresh" content="0; URL=https://holzhaus.github.io/sphinx-multiversion/master/">
- <link rel="canonical" href="https://holzhaus.github.io/sphinx-multiversion/master/">
+ <meta http-equiv="refresh" content="0; URL=https://holzhaus.github.io/sphinx-multiversion/master/index.html">
+ <link rel="canonical" href="https://holzhaus.github.io/sphinx-multiversion/master/index.html">
</head>
</html>