It would function simply: If a player puts their king into check on their own move, if the opponent sees it, they can capture the king on their turn either by physically taking out the king or announcing checkmate on their turn. If the opponent doesn’t see it, the game continues as normal. Likewise, if the opponent puts the king into check, both players can respond on their turn if they notice it, or it’s simply a blundered opportunity.
The classical checkmate ending can be seen as an honorable ending, wherein the player effectively resigns/surrenders, or the player can be dishonorable by drawing out the game by making one more move. I understand the redundancy of this type of ending, but it would add more personable nuance to the game.
This idea can also be extended to stalemates: The only true stalemate would be one where a capturing of the king will not occur in a finite number of moves, e.g. move repetitions, or two sole kings on the board. A stalemate by trapping the king, ie the king has no alternative moves that wouldn’t put it into checkmate, should be a victory rather than a draw — the king would be forced to make a move into check, then the opponent would capture the king on their move if they notice the possibility.
I mean, that’s kinda how it works when playing 1 on 1 with no audience and one player misses moving into check, and the opponent misses too.
And there’s plenty of people that play with no check at all. You don’t have to announce it, it’s on the player to detect and counter.
I prefer check being absent in casual, friendly games unless they’re so friendly that you’re also notifying the other player of missed opportunities or mistakes. Which is, imo, the optimum way to play with friends that aren’t at the same skill level, or are very closely matched to the point that the game is going to drag on when there’s limited time to play.