I don't think this is true, but you won't be able to just grab a shirt off the rack and rock it. Look at Penn Jillette when he was larger, as an example. He was always dressed to the nines. He also strategically incorporated vests into his wardrobe too.

Depending on your shape, a simple undershirt might be slimming enough, or adding shirt stays or shirt garters might help. Worst case you will have to get it tailored. A tshirt is obviously cheaper and easier though, but that signals something.

When I was a fair bit heavier though very active, off the rack shirts didn’t really fit me very well. Tall upper body and very broad shoulders from sports didn’t help. These days I seem much closer to just being able to buy stuff. Bought a new blazer last year and it didn’t need any tailoring which never used to be the case. And shirts work well enough.