According to standard orthography (post-reform) no words are ever written with を, except names and words whose writers exercise "artistic license".
But I don't think in your case it's a misspelling, rather a conscious choice of adhering to pre-reform orthography. (かつお was かつを before the spelling reform.) Opposed to new nonsense uses, like ヲタク (or ワヰン), I think かつを may actually be considered somewhat classy (that would depend on the restaurant though).
Besides restaurants and onsen, ryokan, whatever, を is also reasonably common in names. I have met a number of women called かをる or かをり. (Of course that's because 薫り・香り was かをり before the spelling reform.) Similarly with other obsolete かな like ゑ (e.g. 澄恵 すみゑ) or the repetition mark ゝ (e.g. なゝせ). I have never seen obsolete かな in men's names and I don't really expect to either, because of the soft/traditional connotation of かな. (That said, maybe some parents name their boy レヲン – it's happened to a dog already – just because they can. Ateji for foreign names, as in 零音, as a trend is maybe already on its way out, so next up might be foreign names with obsolete katakana, who knows.)