This is extremely uncommon in my experience, to the point where I've really only heard of it happening secondhand. I've seen people eating alone everywhere from fast food to Michelin star restaurants and done it myself many times. Where have you seen it?

"Table for one" is nearly always accommodated. Bringing a family of 4 or 5 out, and you're much more likely to be denied, or to suffer a long wait.

I mean, you almost always can? During COVID, there was a certain degree of reservations required--min 2 people. But it's pretty darned rare. And I say this as someone who has eaten out solo many hundreds of times. (And tend to eat at fairly decent restaurants.)