The wording is 'expected and recommended', which in legal-jargon is the club believe you should wear one but ultimately it's legally unenforceable.
The only caveat of this is that as part of their rules of entry the club could make it a requirement, but that would be an absolute minefield in many senses.
While fans may be advised/asked to put a mask on, I'd be very surprised if there's any form of enforcement to actually wear one.
Personally, I'll probably wear one when travelling through the concourse and in the bogs, but at my seat or when having a pint - nah.