diff --git a/amaranth/vendor/gowin.py b/amaranth/vendor/gowin.py
index b8f86a2..fcf0df4 100644
--- a/amaranth/vendor/gowin.py
+++ b/amaranth/vendor/gowin.py
@@ -5,8 +5,11 @@ import importlib
 from .. import vendor
 
 
+__all__ = ["GowinPlatform"]
+
+
 def __getattr__(name):
-    if name in ("GowinPlatform",):
+    if name in __all__:
         warnings.warn(f"instead of `{__name__}.{name}`, use `amaranth.vendor.{name}",
                       DeprecationWarning, stacklevel=2)
         return getattr(vendor, name)
diff --git a/amaranth/vendor/intel.py b/amaranth/vendor/intel.py
index 617e627..5788fe1 100644
--- a/amaranth/vendor/intel.py
+++ b/amaranth/vendor/intel.py
@@ -5,8 +5,11 @@ import importlib
 from .. import vendor
 
 
+__all__ = ["IntelPlatform"]
+
+
 def __getattr__(name):
-    if name in ("IntelPlatform",):
+    if name in __all__:
         warnings.warn(f"instead of `{__name__}.{name}`, use `amaranth.vendor.{name}",
                       DeprecationWarning, stacklevel=2)
         return getattr(vendor, name)
diff --git a/amaranth/vendor/lattice_ecp5.py b/amaranth/vendor/lattice_ecp5.py
index 8bf1711..4dbd8b4 100644
--- a/amaranth/vendor/lattice_ecp5.py
+++ b/amaranth/vendor/lattice_ecp5.py
@@ -5,8 +5,11 @@ import importlib
 from .. import vendor
 
 
+__all__ = ["LatticeECP5Platform"]
+
+
 def __getattr__(name):
-    if name in ("LatticeECP5Platform",):
+    if name in __all__:
         warnings.warn(f"instead of `{__name__}.{name}`, use `amaranth.vendor.{name}",
                       DeprecationWarning, stacklevel=2)
         return getattr(vendor, name)
diff --git a/amaranth/vendor/lattice_ice40.py b/amaranth/vendor/lattice_ice40.py
index dd8348d..06a12ab 100644
--- a/amaranth/vendor/lattice_ice40.py
+++ b/amaranth/vendor/lattice_ice40.py
@@ -5,8 +5,11 @@ import importlib
 from .. import vendor
 
 
+__all__ = ["LatticeICE40Platform"]
+
+
 def __getattr__(name):
-    if name in ("LatticeICE40Platform",):
+    if name in __all__:
         warnings.warn(f"instead of `{__name__}.{name}`, use `amaranth.vendor.{name}",
                       DeprecationWarning, stacklevel=2)
         return getattr(vendor, name)
diff --git a/amaranth/vendor/lattice_machxo_2_3l.py b/amaranth/vendor/lattice_machxo_2_3l.py
index 9ccc16b..1ac5a43 100644
--- a/amaranth/vendor/lattice_machxo_2_3l.py
+++ b/amaranth/vendor/lattice_machxo_2_3l.py
@@ -5,8 +5,11 @@ import importlib
 from .. import vendor
 
 
+__all__ = ["LatticeMachXO2Platform", "LatticeMachXO3LPlatform"]
+
+
 def __getattr__(name):
-    if name in ("LatticeMachXO2Platform", "LatticeMachXO3LPlatform"):
+    if name in __all__:
         warnings.warn(f"instead of `{__name__}.{name}`, use `amaranth.vendor.{name}",
                       DeprecationWarning, stacklevel=2)
         return getattr(vendor, name)
diff --git a/amaranth/vendor/quicklogic.py b/amaranth/vendor/quicklogic.py
index 24d6b05..8de7d2b 100644
--- a/amaranth/vendor/quicklogic.py
+++ b/amaranth/vendor/quicklogic.py
@@ -5,8 +5,11 @@ import importlib
 from .. import vendor
 
 
+__all__ = ["QuicklogicPlatform"]
+
+
 def __getattr__(name):
-    if name in ("QuicklogicPlatform",):
+    if name in __all__:
         warnings.warn(f"instead of `{__name__}.{name}`, use `amaranth.vendor.{name}",
                       DeprecationWarning, stacklevel=2)
         return getattr(vendor, name)
diff --git a/amaranth/vendor/xilinx.py b/amaranth/vendor/xilinx.py
index d975344..6862ca5 100644
--- a/amaranth/vendor/xilinx.py
+++ b/amaranth/vendor/xilinx.py
@@ -5,8 +5,11 @@ import importlib
 from .. import vendor
 
 
+__all__ = ["XilinxPlatform"]
+
+
 def __getattr__(name):
-    if name in ("XilinxPlatform",):
+    if name in __all__:
         warnings.warn(f"instead of `{__name__}.{name}`, use `amaranth.vendor.{name}",
                       DeprecationWarning, stacklevel=2)
         return getattr(vendor, name)