We Don't Know
Some letters could have been sent or even those that know the truth could have spoken about it before, just off screen. Remember a few of the people there know the truth. However, it probably doesn't matter now, Jon killed Dany, the Queen and there are those that want to punish him for it; I doubt they'd think a suitable punishment for it would be to name him king.
In the end that knowledge would do him no good but only potential harm. Someone could think the last living Targaryen could still challenge the Iron Throne and with Drogon around might pose too much of a threat so could want to kill him off. Leaving the information secret would be safer for him in exile.
Tyrion: Giving you to the Unsullied would start a war. Letting you walk free would start a war. So our new king has chosen to send you to the Night's Watch.
Jon: There's still a Night's Watch?
Tyrion: The world will always need a home for bastards and broken men. You shall take no wife, hold no lands, father no children. The Unsullied wanted your head of course, but Grey Worm has accepted the justice of a life sentence. Sansa and Arya wanted you freed, but they understand our new king needs to make peace. No one is very happy. Which means it's a good compromise, I suppose.
Game of Thrones, Season 8 Episode 6, "The Iron Throne"
It's also worth noting that Tyrion knew Varys was conspiring against Dany and Jon later found out. Dany also seems to know quite a bit before he is executed so the Unsullied or others could have intercepted the letters/ravens as they were sent out.
To those that would oppose Jon becoming king they would t bring it up in case anyone uses it as leverage for him to be king. Those that would want him as king wouldn’t bring it up in case it puts him in danger as mentioned above.
To address the point of who they were sent to, whilst we don't know again they would have been sent to the various lords and ladies of Westeros. They were of course present so if they knew they were either keeping it secret or the letters never got to them.