This is Mandatory Daytime Protection and is a legal requirement from Ofcom. Any channel that shows age-restricted content during the day must require this, and Sky Comedy, Sky Max, Sky Documentaries & Sky Showcase (perhaps a couple of others now) are the main broadcasters that do this.
It cannot be disabled, and applies across all UK TV platforms, and it won't have happened suddenly as you suggest.
https://www.ofcom.org.uk/consultations-and-statements/category-2/mandatory-daytime-protection gives some background info on the regulations.
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