I'm not sure whether a T will know about BDSM, depends on age and personal preferences I guess. The very basics of it are probably known to most people though.

I've discussed things regarding sex multiple times before. It's always a bit embarrassing, but my T has never reacted in any way to increase that feeling. He talks about it as though it's the most natural thing in the world, which helps a lot. It's the exact same as discussing any other topic. The awkwardness goes away after the first one or two sentences exchanged usually.

It's certainly okay to talk about (as with everything). Whether it is the kind of thing you should discuss is for you to decide, but if you think about it enough to make a post, I think it's a good thing to discuss. I'd personally just bite the bullet and go 'so this weekend I did X', but that might just be my style.