Both are unused outside core:
https://codesearch.wmflabs.org/search/?q=SkinTemplatePreventOtherActiveTabs&i=nope&files=&repos=
https://codesearch.wmflabs.org/search/?q=SkinTemplateTabAction&i=nope&files=&repos=
Let's deprecate them immediately rather than support hooks that add complexity to skin rendering.
# Developer notes
Hooks are how extensions integrate with mediawiki. They can also be registered in LocalSettings.php like so:
```
$wgHooks['SkinTemplatePreventOtherActiveTabs'][] = function (
&$skinTemplate, &res
) {
$res .= 'hello world!';
};
```
For the purpose of this task I suggest understanding how they hooks work and what they do, and then confirm that when marked as deprecated a deprecation warning shows up in MediaWiki.
Use
`
$wgDeprecationWarnings = true;
`
to show deprecation warnings.
Note: https://lists.wikimedia.org/pipermail/wikitech-l/2020-June/093461.html has some notes on how the deprecation process now works. It recently changed so don't rely on the code too much.