It’s more a result of taxation on higher ABV beers.
This ^^^
Wadworth 6X used to be 5.5abv, it’s now around 4.5, IIRC, because of tax. There are still beers around above 5.0, I had a really nice stout from Exeter called ‘Darkness’ the last time I was in the Raven in Bath, that was 5.5. Time before that I had a couple of pints of ‘Tempered Train’, from Pitchfork Ales, which was 5.0. It was lovely, but I was drinking the last of it, the brewery’s closed down. ☹️
As I was drinking in the afternoon, with my pie and mash, it’s a good thing I’ve got my bus pass! 😁