Kings retain, maybe get future picks, and maybe Carter retires after the season. Conditions are probably some combination of GP, pts, how deep Pit goes, how many playoff games Carter plays, how many games he plays/pts he gets in the Final, if Pit wins the Cup, if he plays 50 games next year, if the Pens go to the 3rd rd in 2022, etc. I would assume the conditional 4th in 2023 is easier to get than the conditional 3rd in 2022. I could realistically see the Kings getting nothing unless the Pens win the Cup.
In terms of nostalgia, those days are done for me. There was a while there every player leaving from the golden age hit. Although that was mostly retirements. Too many losses since though. Too many bad losses. Too many bad seasons. That first Cup is damn near a decade ago now. There's obviously no Cup without Carter. That was DL likely trying to save his own job because everything before Carter had not yet added up to contending, and he went all in on Richards.
2nd in goals since he got here. 20 ahead of Brown, in 130 fewer games. Only 9 fewer than Kopitar, in 140 fewer games. Game by game, the top goal scorer on the team since 2012. 3rd in total pts. 3rd in pts%, if you include Lucic's one season.
Carter has been here this long because of the contract. I don't blame him, Blake, or DL. It's just a reality of the cap. It would seem likely that he's thinking about retirement, this would be his last chance, and Pit likely takes little to no risk.