doc: add warning for esm specifier resolution

doc/api/esm.md

@@ -1456,6 +1456,10 @@ _internal_, _conditions_)
 
 > Stability: 1 - Experimental
 
+> **Note: Do not rely on this flag. We plan to remove it once the
+> [Loaders API][] has advanced to the point that equivalent functionality can
+> be achieved via custom loaders.**
+
 The current specifier resolution does not support all default behavior of
 the CommonJS loader. One of the behavior differences is automatic resolution
 of file extensions and the ability to import directories that have an index
@@ -1488,6 +1492,7 @@ success!
 [Import Assertions]: #import-assertions
 [Import Assertions proposal]: https://github.com/tc39/proposal-import-assertions
 [JSON modules]: #json-modules
+[Loaders API]: #loaders
 [Node.js Module Resolution Algorithm]: #resolver-algorithm-specification
 [Terminology]: #terminology
 [URL]: https://url.spec.whatwg.org/

lib/internal/modules/esm/formats.js

@@ -7,8 +7,6 @@ const { getOptionValue } = require('internal/options');
 
 
 const experimentalWasmModules = getOptionValue('--experimental-wasm-modules');
-const experimentalSpecifierResolution =
-  getOptionValue('--experimental-specifier-resolution');
 
 const extensionFormatMap = {
   '__proto__': null,
@@ -43,17 +41,7 @@ function mimeToFormat(mime) {
   return null;
 }
 
-let experimentalSpecifierResolutionWarned = false;
 function getLegacyExtensionFormat(ext) {
-  if (
-    experimentalSpecifierResolution === 'node' &&
-    !experimentalSpecifierResolutionWarned
-  ) {
-    process.emitWarning(
-      'The Node.js specifier resolution in ESM is experimental.',
-      'ExperimentalWarning');
-    experimentalSpecifierResolutionWarned = true;
-  }
   return legacyExtensionFormatMap[ext];
 }
 

lib/internal/modules/esm/resolve.js

@@ -362,6 +362,7 @@ function resolveDirectoryEntry(search) {
 }
 
 const encodedSepRegEx = /%2F|%5C/i;
+let experimentalSpecifierResolutionWarned = false;
 /**
  * @param {URL} resolved
  * @param {string | URL | undefined} base
@@ -376,6 +377,13 @@ function finalizeResolution(resolved, base, preserveSymlinks) {
 
   let path = fileURLToPath(resolved);
   if (getOptionValue('--experimental-specifier-resolution') === 'node') {
+    if (!experimentalSpecifierResolutionWarned) {
+      process.emitWarning(
+        'The Node.js specifier resolution flag is experimental. It could change or be removed at any time.',
+        'ExperimentalWarning');
+      experimentalSpecifierResolutionWarned = true;
+    }
+
     let file = resolveExtensionsWithTryExactName(resolved);
 
     // Directory

test/es-module/test-esm-specifiers-legacy-flag-warning.mjs

@@ -0,0 +1,24 @@
+import { mustCall } from '../common/index.mjs';
+import { fileURL } from '../common/fixtures.mjs';
+import { match, strictEqual } from 'assert';
+import { spawn } from 'child_process';
+import { execPath } from 'process';
+
+// Verify experimental warning is printed
+const child = spawn(execPath, [
+  '--experimental-specifier-resolution=node',
+  '--input-type=module',
+  '--eval',
+  `import ${JSON.stringify(fileURL('es-module-specifiers', 'package-type-module'))}`,
+]);
+
+let stderr = '';
+child.stderr.setEncoding('utf8');
+child.stderr.on('data', (data) => {
+  stderr += data;
+});
+child.on('close', mustCall((code, signal) => {
+  strictEqual(code, 0);
+  strictEqual(signal, null);
+  match(stderr, /ExperimentalWarning: The Node\.js specifier resolution flag is experimental/);
+}));