There's a 4K 4:3 remaster streaming on HBO Max. It wasn't exactly well advertised (certainly not to the extent that it has been hard to miss this Mad Men advertising), but it exists and is a good way to watch the show. Feels a bit less cinematic not being in 16:9, but it looks good other than that because the show was shot to be 4:3 safe (as that was still the most common TV at the time).