Not attempting to contradict any of the other answers, which in my humble opinion have nailed most of the issue, but none have addressed the question of companies and official forms.
A handwritten letter to a company in Sütterlin would be very weird. Unless you’re really 90+, learnt it at school and never used any other handwriting, the company would (as soon as it has figured out the meaning of the text, because they likely can’t read it) ask themselves what actually just happened. It’s anybody’s guess what they would think, but it’s not worth the trouble — especially since computers with printers are now widely available.
Not even bank transfer forms would qualify. My grandmother once tried to fill one in — frankly I never saw the form, just read on the account statement »50 Mark für Jan, 50 für Kalus, Rest unleserlich«. Yes, granted, she had pain in her fingers, but she likely also wrote in a very old-fashioned handwriting.
On official German forms (and likely Austrian and Swiss ones, too), Sütterlin would, in my opinion, have to be accepted. Reasoning: There are those people 90+ that have never used any other handwriting. It might take a little longer for the civil servant to read it, (see above) but they can’t really reject it.
However, if the official thinks you’re pulling their leg, things might change, and you might still be required to write it properly. Remember that Sütterlin hasn’t been taught as a handwriting for over 70 years.