ugh is it so hard for sm manga to have a consensual relationship? At least define a fucking safeword...
safewords are fucking bullshit. They were created as a roleplay tool, so one could say "stop" and "no" without meaning it, and then grew too large and gained too much 'importance'. To the point that it gets used as a talisman by abusers. "Well, did she safeword?" i've heard from limit-breakers, in the same tone of voice as "What was she wearing?" in regards to rape. Give me clear communication any day, fuck the "safeword" bull shit at this point. It should not be called on the SUB to keep track of her limits like that, when being put into altered headspaces!
BTW, there technically WAS a 'safeword' in Chapter 4. it was "you can take off the collar any time, it is not mine to take it off of you". There. That's the "if you want it to stop, this is how to stop it".