Someone at Apple should be slapped upside the head for this.
There's a thread or two on Apple Discussions Forum where people are livid about this preference omission. For vision impaired people this is a real kick in the butt.
Apple needs to rectify this asap.
Hopefully Apple will fix this with a new Safari update, you can remedy this somewhat by going to Preferences, Advanced, and clicking to a higher zoom level however the 1st level above 100 is 115% which makes the font larger than if you had the original settings of not smaller than 14, in addition there are certain parts of a webpage's font that won't appear larger even with the zoom level higher.
But if they are only going to allow a zoom level, it needs to be more fine tuned with levels of 105/110 etc.
Someone on Twitter reached out to the Apple Support Twitter and they responded by suggesting Zoom but as the Twitter use wrote, Zoom also seems to decrease the sharpness, for people that rely on a certain font, using the Zoom function causes eye strain. I will switch to Chrome until Apple can fix this. I just wish I could import my favorites bar and keychain passwords in a seamless fashion.