diff --git a/.github/workflows/validate.yml b/.github/workflows/validate.yml
index f221f07b..ee05762f 100644
--- a/.github/workflows/validate.yml
+++ b/.github/workflows/validate.yml
@@ -55,7 +55,10 @@ jobs:
           spaze/phpstan-disallowed-calls
 
       - if: matrix.php-version == '8.0'
-        run: composer remove --dev --no-update rector/rector
+        run: >
+          composer remove --dev --no-update
+          rector/rector
+          spaze/phpstan-disallowed-calls
 
       - if: matrix.php-version == '7.4' || matrix.php-version == '8.0'
         run: rm -rf src/Enum tests/Enum src/FluidXPlate/Scalars src/Microplate/Scalars tests/FluidXPlate/Scalars tests/Microplate/Scalars
@@ -102,7 +105,10 @@ jobs:
           spaze/phpstan-disallowed-calls
 
       - if: matrix.php-version == '8.0'
-        run: composer remove --dev --no-update rector/rector
+        run: >
+          composer remove --dev --no-update
+          rector/rector
+          spaze/phpstan-disallowed-calls
 
       - if: matrix.php-version == '7.4' || matrix.php-version == '8.0'
         run: rm -rf src/Enum tests/Enum src/FluidXPlate/Scalars src/Microplate/Scalars tests/FluidXPlate/Scalars tests/Microplate/Scalars
diff --git a/composer.json b/composer.json
index 9f3cb7cb..aa6cfec6 100644
--- a/composer.json
+++ b/composer.json
@@ -39,7 +39,7 @@
     "phpstan/phpstan-strict-rules": "^1 || ^2.0.3",
     "phpunit/phpunit": "^9.6.22 || ^10.5.45 || ^11.5.10 || ^12.0.5",
     "rector/rector": "^1.2.10 || ^2.0.9",
-    "spaze/phpstan-disallowed-calls": "^2 || ^4.4",
+    "spaze/phpstan-disallowed-calls": "^4.4",
     "thecodingmachine/phpstan-safe-rule": "^1.2.0"
   },
   "suggest": {
diff --git a/phpstan/include-by-php-version.php b/phpstan/include-by-php-version.php
index 6038baf5..c4cddb98 100644
--- a/phpstan/include-by-php-version.php
+++ b/phpstan/include-by-php-version.php
@@ -2,9 +2,7 @@
 
 $includes = [];
 
-// PHP 8.0+ can use rules.neon (requires spaze/phpstan-disallowed-calls)
-// PHP 7.4 has this package removed via composer, so skip rules.neon
-if (version_compare(PHP_VERSION, '8.0', '>=')) {
+if (class_exists(\Spaze\PHPStan\Rules\Disallowed\DisallowedHelper::class)) {
     $includes[] = __DIR__ . '/../rules.neon';
 }
 
diff --git a/rules.neon b/rules.neon
index f354ac38..51c95af6 100644
--- a/rules.neon
+++ b/rules.neon
@@ -14,6 +14,10 @@ parameters:
   - function: 'clock()'
     message: 'it is just meant for debugging.'
     errorTip: 'remove debugging code.'
+  disallowedClasses:
+  - class: 'Carbon\CarbonImmutable'
+    message: 'mixing mutable and immutable Carbon semantics makes code hard to reason about.'
+    errorTip: 'use Carbon\Carbon instead. See https://gitlab.mll/documentation/coding-guidelines/-/issues/313'
   disallowedStaticCalls:
   - method: 'Carbon\Carbon::create()'
     message: 'it ignores errors.'