The SkinTemplateOutputPageBeforeExec hook will be soft deprecated in T60137
Preferred hooks are listed in https://www.mediawiki.org/wiki/User:Jdlrobson/Skins_for_extension_developers
In 1.36 the Vector skin will no longer run this hook and depending on migration efforts the hook may be hard deprecated.
Let me know if you have any questions or need any help with this migration. I'll be happy to help you!