docs: added to migration guide

Author: DanRibbens

docs/migration-guide/v4.mdx

@@ -325,6 +325,31 @@ export const myRichTextAdapter = (): RichTextAdapterProvider => {
 
 The `transactionIDPromise` property on `PayloadRequest` has been removed. It was deprecated and unused — `transactionID` (which already supports `Promise<number | string>`) is the correct property to use for both resolved values and pending transactions.
 
+### `defaults` config export removed from `payload` ([#17103](https://github.com/payloadcms/payload/pull/17103))
+
+The `defaults` object exported from `payload` has been removed. It was deprecated because it was a single shared object — mutating it (or any config that used it as a base) leaked changes into every other consumer of the defaults.
+
+```diff
+- import { defaults } from 'payload'
+-
+- const config = {
+-   ...defaults,
+-   // your overrides
+- }
+```
+
+You no longer need to apply defaults yourself. `buildConfig` already fills in every default value during config initialization, so pass only the properties you want to override:
+
+```ts
+import { buildConfig } from 'payload'
+
+export default buildConfig({
+  // only your overrides — defaults are applied automatically
+})
+```
+
+If you depended on reading a specific default value at runtime, read it from the sanitized config returned by `buildConfig` (or `payload.config`) instead of from the static `defaults` object.
+
 ### Plugin Import/Export: `toCSV` and `fromCSV` removed
 
 The `toCSV` and `fromCSV` field options in `custom['plugin-import-export']` have been removed. Use `hooks.beforeExport` and `hooks.beforeImport` instead — they work for both CSV and JSON formats.