If you think of any (societal) issue you care about, there's a good chance it would get solved with that tiny change
The specificity required of legislation to enact such a thing would be ridiculously un-tiny.
But, yes, it should be done, it should exist, it is the right thing to do, it is worth the effort.
A delicate issue I see is how to handle personal matters that the public don't need to know.
Anyhow, you might think that it's hard to know when someone is really lying vs just being uninformed, but in truth in the long term most lies become apparent; while you couldn't prevent every single lie, you'd reduce them enormously, in my opinion; even in countries where the president doesn't lie almost constantly.
But, how's that's going for us recently...
Not to mention its absence in a president's second term...