Obviously the series conveniently forgets about safewords and talking anything through before sex despite it's semi realism (and it magically works for them perfectly, which, great) but I really like how the story always returns to how they care about each other - in juxtaposition with the sadism/masochism AND the genuine callousness of how Io's treated by the rest of the world, most of all Hase and her mother
IIRC Misa doesn't have prior BDSM experience, and Io certainly doesn't, so they're making it up as they go. You can't forget something you never knew.
I also occasionally see claims that safewords aren't as big a part of Japan's BDSM culture. A source I lost said they relied on "just pay attention"; this one is more negative: "Never have I ever heard Japanese people in the scene talk about considerations related to BDSM best practices in the States, such as negotiation or safewords."
https://www.sharingmytruth.com/the-japanese-bdsm-scene doesn't say "Japan doesn't have safe words" but Speaker A describes a relationship they were that didn't have safe words. "I didn't necessarily have a death wish, but I felt that I needed to be punished... But who was consenting? Not me, my subconscious, that wanted to be beaten up and punished. So although there was consent, inherent consent, it was coming from a very dark place."
Sound like Io?