Received request for horizontal site design. What's the general consensus these days?

Hello! I'm looking for some self-affirming answers, but I'm willing to hear out the other side of things and I will stand corrected if I'm wrong ;)

A large non-profit wants a new website and they LOVE a random horizontal scrolling site they found. They want us to do something like that. It's not even a creative use of horizontal scrolling, its just a full mouse takeover and static scrolling using the system scrollbar and everything.

Everyone at my firm hates it, but ultimately we will do what the client wants if we have to. However we want to try to explain to them why it's not a good idea. Back in the old days, this was a no-no because of the early gen screen readers for disability and all that. But is that still the case? Are there any other reasons I can point to that have facts to support why it's not suggested? I have good instincts and some clients listen to that, but I'm trying to do my due diligence. Google helps a little but most of the info that comes up is HOW to do a horizontal scroll.

Am I totally wrong and horizontal is the new thing going on? I am ok with small elements or sections of a longer page having some kind of horizontal scrolling element, but not the entire site!

Thanks in advance!