Document deprecation of `marked_for_deletion_at` attribute

doc/api/projects.md

@@ -162,7 +162,7 @@ When the user is authenticated and `simple` is not set this returns something li
     "merge_method": "merge",
     "autoclose_referenced_issues": true,
     "suggestion_commit_message": null,
-    "marked_for_deletion_at": "2020-04-03",
+    "marked_for_deletion_at": "2020-04-03", // to be deprecated in GitLab 13.0 in favor of marked_for_deletion_on
     "marked_for_deletion_on": "2020-04-03",
     "statistics": {
       "commit_count": 37,
@@ -287,6 +287,9 @@ When the user is authenticated and `simple` is not set this returns something li
 ]
 ```
 
+NOTE: **Note:**
+For users on GitLab [Silver, Premium, or higher](https://about.gitlab.com/pricing/) the `marked_for_deletion_at` attribute will be deprecated in GitLab 13.0 in favor of the `marked_for_deletion_on` attribute.
+
 Users on GitLab [Starter, Bronze, or higher](https://about.gitlab.com/pricing/) will also see
 the `approvals_before_merge` parameter:
 
@@ -408,7 +411,7 @@ This endpoint supports [keyset pagination](README.md#keyset-based-pagination) fo
     "merge_method": "merge",
     "autoclose_referenced_issues": true,
     "suggestion_commit_message": null,
-    "marked_for_deletion_at": "2020-04-03",
+    "marked_for_deletion_at": "2020-04-03", // to be deprecated in GitLab 13.0 in favor of marked_for_deletion_on
     "marked_for_deletion_on": "2020-04-03",
     "statistics": {
       "commit_count": 37,
@@ -874,7 +877,7 @@ GET /projects/:id
   "service_desk_address": null,
   "autoclose_referenced_issues": true,
   "suggestion_commit_message": null,
-  "marked_for_deletion_at": "2020-04-03",
+  "marked_for_deletion_at": "2020-04-03", // to be deprecated in GitLab 13.0 in favor of marked_for_deletion_on
   "marked_for_deletion_on": "2020-04-03",
   "statistics": {
     "commit_count": 37,

ee/changelogs/unreleased/212937-doc-changes-for-marked-for-deletion-at-deprecation.yml

+---
+title: Document planned deprecation of 'marked_for_deletion_at' attribute in Projects
+  API in GitLab 13.0
+merge_request: 28993
+author:
+type: deprecated