excel.new has a "weird" TLD, no mention of Microsoft in the domain and asks me to auth with my Microsoft account before I can even see the site. If I came across this in the wild, I'd for sure think this is some phishing or generally bad website.
The .new TLD is pretty nifty. sheets.new gets you the same thing as excel.new, but for Google.
Using X.new seems like a bad idea, there is no way for me to understand if it's actually a official project by the companies themselves or just a random developer who has set that up. Maybe one day it'll ask someone for auth, they don't notice it's at microsoftweb.com instead of microsoftonline.com and get phished.
Really hard to trust random domains when a simple excel.office.com/new would do just as fine.
What is wrong with excel.office.com with a version toggle?