And I think there's an argument that we do!

The same page I linked says:

We recommend you don’t use profile pictures, filters, or frames that support a candidate or party, to avoid the risk of committing an offence if you post using the picture or frame on election day. Photos, filters and frames saying you’ve voted or which encourage others to vote are fine.

I feel like saying who you voted for could fit into the same sort of offence if you caught the enforcer (Police?) on a bad day.