It's ok if they don't support it when displaying mail, but by just setting that flag on sending, people with good clients will get the best possible experience
Email is ancient; it still works; it usually does not need round CSS corners nor bold HTML stuff.
"Modern" clients gives you utter shit like WebTV and midi-file signatures.
Seriously?! Is this a thing? Please tell me that was hyperbole.
Also, this is not just about compatibility with old clients, but also about compatibility with old emails. If you redefine the meaning of previously used email syntax, that changes how old emails get displayed, which sucks if you have an email archive that goes back more than a decade.
But the good thing about format=flowed is that even if a client doesn't support it for display, it'll still look good (on desktops)