I was born in San Francisco in 1950, and have lived in or in close proximity to it (Berkeley, Oakland, Daly City) all my life.

The phrase "don't call it Frisco" goes back further than I do, and apparently was often the first thing a new resident was told after moving here. From what I was hearing and reading, i think it was considered insulting back then, more than simply ignorant, if someone did it consistently. (Herb Caen, the "three-dot" newspaper columnist who was considered our most influential and most popular writer, would frequently title part of his dally collection of short items "Don't Call It Frisco Roulette".)

In 1961, one of the two local Top 40 radio stations brought in a new morning drive DJ from Los Angeles, Don MacKinnon, who told his listeners on the first day that he was going to call it Frisco until someone gave him a reason not to. (In the last hour, he got a phone call from a woman who told him he was denigrating Saint Francis of Assisi, for whom the local Spanish mission and eventually the surrounding community was named back in the 19th century. Only time I've ever heard that, FWIW.) He didn't last very long before returning to SoCal (a term not in general usage back then, at least up here).

I'm told there were Millennials who were told the same thing about "San Fran", as a nickname to avoid. I'm not sure anyone my age objected to it; I didn't, though S F (Ess Eff) was preferable.

The real nickname used by San Franciscans, one that went back further than I did, was "the City". (In the days before postal codes -- ZIP codes, here in the U.S -- the story goes that someone in a small Midwestern town about 2000 miles away tried to pay a local utility bill by mailing a check to "Water Dept., City" and it turned up in San Francisco.) I think the name really does go back to the days after the 1849 Gold Rush when we were basically the only large city on the West Coast, but in any case, residents of the Bay Area were calling it the City for a long time,

The name got some publicity in the mid-1960s when the San Francisco Warriors NBA basketball team (later Golden State) introduced new uniforms with that, along with a bridge on the front and a cable car surrounding the uniform number on the back. I'm not sure the term was capitalized until the Warriors did that, though.

Eric