This depends on location. There was a thing a few years ago of people taking selfies with their ballots and posting them publicly. This becomes a very serious concern when the secret ballot - and preventing external pressures to vote a certain way - comes into question.

Some places consider it free speech to announce (and prove) who you’re voting for; others view it as an invasion of the ballot box. I don’t believe it’s ever been reviewed by the courts.

ETA: fixed autocorrect again