Remove strike deprecation and add added-in annotation

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

changelogs/client_server/newsfragments/1732.clarification

@@ -0,0 +1 @@
+Document the deprecation policy of HTML tags.

changelogs/client_server/newsfragments/1732.feature

@@ -1 +0,0 @@
-Deprecate the `strike` HTML tag and introduce the `s` tag as a possible replacement.

content/client-server-api/modules/instant_messaging.md

@@ -48,21 +48,13 @@ tags to permit, denying the use and rendering of anything else, is:
 `s`, `code`, `hr`, `br`, `div`, `table`, `thead`, `tbody`, `tr`,
 `th`, `td`, `caption`, `pre`, `span`, `img`, `details`, `summary`.
 
-
 {{% boxes/note %}}
+{{% added-in v="1.10" %}}
 HTML features MAY be deprecated and replaced by their modern equivalent without
 requiring a [Spec Change Proposal](/proposals) when they are deprecated in the
 WHATWG HTML Living Standard.
 {{% /boxes/note %}}
 
-{{% boxes/note %}}
-{{% changed-in v="1.10" %}}
-
-The `strike` tag is deprecated. Clients MUST stop sending new messages using
-this tag and replace it with `s` or `del`.
-{{% /boxes/note %}}
-
-
 Not all attributes on those tags should be permitted as they may be
 avenues for other disruption attempts, such as adding `onclick` handlers
 or excessively large text. Clients should only permit the attributes