Immutables with literal values are pure.

test/libsolidity/syntaxTests/immutable/unrelated_reading.sol

@@ -1,7 +1,7 @@
 contract C {
     uint immutable x = 1;
 
-    function readX() internal view returns(uint) {
+    function readX() internal pure returns(uint) {
         return x + 3;
     }
 }

test/libsolidity/syntaxTests/viewPureChecker/immutable.sol

@@ -5,4 +5,3 @@ contract B {
     }
 }
 // ----
-// TypeError 2527: (100-101): Function declared as pure, but this expression (potentially) reads from the environment or state and thus requires "view".