I think if you are telling people that your marriage is over and it is juts a matter of the divorce making its way through the system then it is all on you. I did start dating before my divorce was final. My husband decided to get a sex change operation and had been pretty much over for years before we decided to end things. I very much doubt anyone was shocked or upset about our divorce and I would have taken a very dim view of someone telling me I wasn't allowed to date yet because they hadn't processed our divorce yet. I also felt no need to pretend there was a chance to mend things or to deny myself a chance at happiness just because the courts are slow. So I am not willing to condemn other people for doing the same thing.
The thing is, it's not that anybody is telling you not to date.
It's that Etiquette says, "don't make other people uncomfortable by putting them in a position of feeling that they have to choose between, on the one hand, being disrespectful to the institution of marriage or to all the parties involved, or on the other, being disrespectful to you by not acknowledging the important things in your life."
It's not about the state of your heart, or your STBX-spouse's heart. It's about how you make other people feel by your public declarations.
So when everyone around you is aware of how thoroughly your marriage is over, you don't need to be quite as discreet. And you might need to be more discreet with people who aren't as close to you (or maybe less discreet).
In the old days, it was easy: You did not, ever, push your new relationship
into other people's view until the divorce was final.
It's not as easy now, because divorce is easier (and also harder, actually), and we don't have the same universal standards that we once did.
So now? I don't know. I'd personal argue that if you want to be believed to have "good form," you don't change your Facebook relationship
status until your divorce is final--or at least close.