deprecation announcement for oauth implicit grant

data/deprecations/15-0-oauth.yml

+- name: "OAuth implicit grant" # The name of the feature to be deprecated
+  announcement_milestone: "14.0" # The milestone when this feature was first announced as deprecated.
+  announcement_date: "2021-06-22" # The date of the milestone release when this feature was first announced as deprecated. This should almost always be the 22nd of a month (YYYY-MM-22), unless you did an out of band blog post.
+  removal_milestone: "15.0" # The milestone when this feature is planned to be removed
+  removal_date: 2022-05-22 # The date of the milestone release when this feature was first announced as deprecated. This should almost always be the 22nd of a month (YYYY-MM-22), unless you did an out of band blog post.
+  breaking_change: yes # If this deprecation is a breaking change, set this value to true
+  body: | # Do not modify this line, instead modify the lines below.
+    The OAuth implicit grant authorization flow will be removed in our next major release, GitLab 15.0. Any applications that use OAuth implicit grant should switch to alternative [supported OAuth flows](https://docs.gitlab.com/ee/api/oauth2.html).
+# The following items are not published on the docs page, but may be used in the future.
+  stage: # Manage
+  tiers: # (optional - may be required in the future) An array of tiers that the feature is available in currently.  e.g., [Free, Silver, Gold, Core, Premium, Ultimate]
+  issue_url: # https://gitlab.com/gitlab-org/gitlab/-/issues/344609
+  documentation_url: # (optional) This is a link to the current documentation page
+  image_url: # (optional) This is a link to a thumbnail image depicting the feature
+  video_url: # (optional) Use the youtube thumbnail URL with the structure of https://img.youtube.com/vi/UNIQUEID/hqdefault.jpg

doc/update/deprecations.md

@@ -58,6 +58,18 @@ We encourage customers currently using NFS for Git repositories to plan their mi
 
 **Planned removal milestone: 15.0 (2022-05-22)**
 
+### OAuth implicit grant
+
+WARNING:
+This feature will be changed or removed in 15.0
+as a [breaking change](https://docs.gitlab.com/ee/development/contributing/#breaking-changes).
+Before updating GitLab, review the details carefully to determine if you need to make any
+changes to your code, settings, or workflow.
+
+The OAuth implicit grant authorization flow will be removed in our next major release, GitLab 15.0. Any applications that use OAuth implicit grant should switch to alternative [supported OAuth flows](https://docs.gitlab.com/ee/api/oauth2.html).
+
+**Planned removal milestone: 15.0 (2022-05-22)**
+
 ## 14.2
 
 ### Release CLI distributed as a generic package