diff --git a/include/pybind11/detail/class.h b/include/pybind11/detail/class.h
index 1cd9af0bd..21e966cfe 100644
--- a/include/pybind11/detail/class.h
+++ b/include/pybind11/detail/class.h
@@ -226,12 +226,14 @@ extern "C" inline void pybind11_meta_dealloc(PyObject *obj) {
                 local_internals.registered_types_cpp.erase(tinfo->cpptype);
             } else {
                 internals.registered_types_cpp.erase(tindex);
+#if PYBIND11_INTERNALS_VERSION >= 12
                 internals.registered_types_cpp_fast.erase(tinfo->cpptype);
                 for (const std::type_info *alias : tinfo->alias_chain) {
                     auto num_erased = internals.registered_types_cpp_fast.erase(alias);
                     (void) num_erased;
                     assert(num_erased > 0);
                 }
+#endif
             }
             internals.registered_types_py.erase(tinfo->type);
 
diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index 11a2ee4c9..534751153 100644
--- a/include/pybind11/detail/internals.h
+++ b/include/pybind11/detail/internals.h
@@ -39,11 +39,11 @@
 /// further ABI-incompatible changes may be made before the ABI is officially
 /// changed to the new version.
 #ifndef PYBIND11_INTERNALS_VERSION
-#    define PYBIND11_INTERNALS_VERSION 12
+#    define PYBIND11_INTERNALS_VERSION 11
 #endif
 
-#if PYBIND11_INTERNALS_VERSION < 12
-#    error "PYBIND11_INTERNALS_VERSION 12 is the minimum for all platforms for pybind11v3."
+#if PYBIND11_INTERNALS_VERSION < 11
+#    error "PYBIND11_INTERNALS_VERSION 11 is the minimum for all platforms for pybind11v3."
 #endif
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
@@ -242,12 +242,14 @@ struct internals {
     pymutex mutex;
     pymutex exception_translator_mutex;
 #endif
+#if PYBIND11_INTERNALS_VERSION >= 12
     // non-normative but fast "hint" for registered_types_cpp. Meant
     // to be used as the first level of a two-level lookup: successful
     // lookups are correct, but unsuccessful lookups need to try
     // registered_types_cpp and then backfill this map if they find
     // anything.
     fast_type_map<type_info *> registered_types_cpp_fast;
+#endif
 
     // std::type_index -> pybind11's type information
     type_map<type_info *> registered_types_cpp;
@@ -273,6 +275,9 @@ struct internals {
     PyObject *instance_base = nullptr;
     // Unused if PYBIND11_SIMPLE_GIL_MANAGEMENT is defined:
     thread_specific_storage<PyThreadState> tstate;
+#if PYBIND11_INTERNALS_VERSION <= 11
+    thread_specific_storage<loader_life_support> loader_life_support_tls; // OBSOLETE (PR #5830)
+#endif
     // Unused if PYBIND11_SIMPLE_GIL_MANAGEMENT is defined:
     PyInterpreterState *istate = nullptr;
 
@@ -353,6 +358,7 @@ struct type_info {
     void *(*module_local_load)(PyObject *, const type_info *) = nullptr;
     holder_enum_t holder_enum_v = holder_enum_t::undefined;
 
+#if PYBIND11_INTERNALS_VERSION >= 12
     // When a type appears in multiple DSOs,
     // internals::registered_types_cpp_fast will have multiple distinct
     // keys (the std::type_info from each DSO) mapped to the same
@@ -363,6 +369,7 @@ struct type_info {
     // nb_alias_chain` added in
     // https://github.com/wjakob/nanobind/commit/b515b1f7f2f4ecc0357818e6201c94a9f4cbfdc2
     std::forward_list<const std::type_info *> alias_chain;
+#endif
 
     /* A simple type never occurs as a (direct or indirect) parent
      * of a class that makes use of multiple inheritance.
diff --git a/include/pybind11/detail/type_caster_base.h b/include/pybind11/detail/type_caster_base.h
index 21b7f0950..b0c59e113 100644
--- a/include/pybind11/detail/type_caster_base.h
+++ b/include/pybind11/detail/type_caster_base.h
@@ -227,26 +227,32 @@ inline detail::type_info *get_global_type_info_lock_held(const std::type_info &t
     // next time.
     detail::type_info *type_info = nullptr;
     auto &internals = get_internals();
+#if PYBIND11_INTERNALS_VERSION >= 12
     auto &fast_types = internals.registered_types_cpp_fast;
+#endif
     auto &types = internals.registered_types_cpp;
+#if PYBIND11_INTERNALS_VERSION >= 12
     auto fast_it = fast_types.find(&tp);
     if (fast_it != fast_types.end()) {
-#ifndef NDEBUG
+#    ifndef NDEBUG
         auto types_it = types.find(std::type_index(tp));
         assert(types_it != types.end());
         assert(types_it->second == fast_it->second);
-#endif
+#    endif
         return fast_it->second;
     }
+#endif // PYBIND11_INTERNALS_VERSION >= 12
 
     auto it = types.find(std::type_index(tp));
     if (it != types.end()) {
+#if PYBIND11_INTERNALS_VERSION >= 12
         // We found the type in the slow map but not the fast one, so
         // some other DSO added it (otherwise it would be in the fast
         // map under &tp) and therefore we must be an alias. Record
         // that.
         it->second->alias_chain.push_front(&tp);
         fast_types.emplace(&tp, it->second);
+#endif
         type_info = it->second;
     }
     return type_info;
diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index 8bd62c85c..91b38d91e 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1692,7 +1692,9 @@ protected:
                 local_internals.registered_types_cpp[rec.type] = tinfo;
             } else {
                 internals.registered_types_cpp[tindex] = tinfo;
+#if PYBIND11_INTERNALS_VERSION >= 12
                 internals.registered_types_cpp_fast[rec.type] = tinfo;
+#endif
             }
 
             PYBIND11_WARNING_PUSH
@@ -2199,7 +2201,9 @@ public:
                     type_info *const val
                         = internals.registered_types_cpp[std::type_index(typeid(type))];
                     internals.registered_types_cpp[std::type_index(typeid(type_alias))] = val;
+#if PYBIND11_INTERNALS_VERSION >= 12
                     internals.registered_types_cpp_fast[&typeid(type_alias)] = val;
+#endif
                 }
             });
         }