From the eagle-eyed @jonseddon 🦅 👁 :

"I just noticed that the text inside the HTML <title> tag for the Iris 2.4 documentation at https://scitools.org.uk/iris/docs/v2.4.0/gallery.html says “Thumbnail gallery — Iris 2.2.0 documentation” rather than 2.4. It’s correct in the equivalent page in the version 2.3 documentation. I’ve not done an extensive search of all pages, but it looks like there’s a similar problem in the version 2.4 User Guide at https://scitools.org.uk/iris/docs/v2.4.0/userguide/index.html."

This appears to be matched by the version drop-down in the top right, too.