🩺 Assert FAN_SOFT_PWM where required (#23383, #23477)

Marlin/src/inc/SanityCheck.h

@@ -1995,8 +1995,12 @@ static_assert(Y_MAX_LENGTH >= Y_BED_SIZE, "Movement bounds (Y_MIN_POS, Y_MAX_POS
   #endif
 #endif
 
-#if HAS_FAN0 && CONTROLLER_FAN_PIN == FAN_PIN
-  #error "You cannot set CONTROLLER_FAN_PIN equal to FAN_PIN."
+#if HAS_FAN0
+  #if CONTROLLER_FAN_PIN == FAN_PIN
+    #error "You cannot set CONTROLLER_FAN_PIN equal to FAN_PIN."
+  #elif ENABLED(FAN_SOFT_PWM_REQUIRED) && DISABLED(FAN_SOFT_PWM)
+    #error "FAN_SOFT_PWM is required. Enable it to continue."
+  #endif
 #endif
 
 #if ENABLED(USE_CONTROLLER_FAN)