We all feeling good? Yeah? Nice and positive atmosphere? Cool. We deserve it.
A lot of us reading know what it's like to be in a room RPing and know that another character is there invisible, listening. Back in the day this can mean that the conversation changes, quite suddenly and suspiciously, from undermining one character's plans to the movie night.
Does this still happen? I don't know; the last game I've been on with this as a consideration we had to open a job with staff and wait a few days for an answer. But in talking who knows who else is a vampire or a witch, that does seem to be dead in the community. I'd give it about a decade in the ground at this point.
But there's more to OOC Masq then knowing the potential power set of characters that your character might have reason to play against. Ghost's example is one. Mine is another. I don't think that's dead, but I also don't think it's unhealthy.
But it's not cut and dry. "You can't know I'm here. I can't help that there's no code supporting invisibility." Things that we decide our characters know IC based on decisions we make OOC affect other characters. This is a tool, intrinsically nether good or bad.
The key to me is if it's fun and reasonable. Is it plausible? Cool. Is it reasonable in the IC nature of the game? Awesome!
I know there are people already lining up their responses with those like "that's never happened to me" or "I'm not responsible for the fun of others" or "just don't make a game where that's possible". Pretend this post doesn't exist because it doesn't affect you, your play style, or the kind of games that you play. Here's your cookie and feeling of self-worth. I'm honestly kind of jealous.
For the rest of us? Let it roll!