Plenty of road bikes from the last few decades can't fit 30mm tires. A few have trouble even fitting 25s.
That's more or less why "gravel bike" has been able to become a phrase: we're coming out of a bizarre period where nearly all road bikes were aggressively designed to be
non-gravel bikes. Go back a few decades, and it was pretty normal for road bikes to fit 32mm tires; plenty even came stock with such widths. Really old racing photos are often fun:
Depending on the gravel, though, 30mm tires can still be very marginal. If the surface is very rough or loose, you might need to pump them down to extreme drop, which in addition to very poor rolling resistance means that you're exposing your sidewalls a lot to the riding surface. That then requires beefing up the sidewalls, which further increases rolling resistance...
Not that road bikes with even bigger tire clearances and itsy-bitsy granny gears are anything new either...
That's probably the main thing that makes me skeptical of the "gravel bike" moniker. A lot of cyclists traditionally tend to feel that anything that makes accommodations for some rougher riding is severely compromised as a road bike. "Gravel bike" feels like it kind of embraces this outlook.