If you are not a skin developer you can disregard this message.
In MediaWiki 1.39, skins started triggering a deprecation notice until they ceded the calling of the OutputPage::headElement and OutputPage::tailElement functions e.g. the output of the <html> tags, the <head> element and any code rendered between the closing </body> tag and </html> element. The intention behind this was for skins to provide a predictable environment in which extensions could operate (e.g. assuming access to ResourceLoader and MediaWiki config variables for example).
Since these deprecation notices appeared, all skins that we are aware of have been updated to meet these requirements.
In MediaWiki 1.43, we have therefore dropped support for rendering the head element.
If you have a private skin that does not publish its code publicly and/or is not listed on MediaWiki.org, it's advised that you check your logs for deprecation warnings before upgrading to the new MediaWiki version to ensure your skin is ready. Failure to do this could potentially lead to broken page views.
Feel free to reply to this message if you need any help.