To the original-ish point though, boundary setting is not so much about what you do TO or FOR other people. It's more deciding in advance how YOU will react, when someone's behavior starts to feel violating to you, before it gets to the point of being an actual violation..
This is why I don't use the same thought process behind boundary setting that I do towards someone being abusive. That can happen out of left field that someone decides to violate you. It can happen from "friends" or strangers. Once that has been done, I think boundaries are kind of beside the point. They do not prevent abusers from abusing. I think that should be made super clear.
Boundaries will /not prevent abusive people from continuing on with their abusive behavior/. What they can do though is help increase your comfort because you've come up with a plan of action when things start to brush up against issues/behaviors that make you feel extremely uncomfortable. Not that they'll make you immune from having to deal with it. Like you really cannot escape that in interacting with other humans.
Deciding that you will speak up when you feel that you are being belittled, or that you will page or send a mail privately to someone if they are making ooc comments towards you that are unkind or seem to be belittling or that you will check in periodically with someone for reassurance--these are all great and healthy boundaries. They're about actions that you can take to protect yourself and to check yourself before an issue becomes something more than that.
Deciding in the moment that if someone that you have RPed with for months and months about a certain subject or in a certain line of play ask you for it one more time you will demand they change or cut ties since obviously they're not getting the hints you've dropped along the way that you really don't like it--that is a reaction (and probably an understandable one) but you have been violating your own boundary by never directly stating that you don't want this to happen and want it to stop. You should probably expect some surprise/shock/bad feelings from that person because it is hard to be told something you thought the other person liked they've hated this whole time, and they may wonder what else you've been hiding. It may lead to a breach of trust that they feel happened on your end too. It's not going to be comfortable for either of you, but there's no reason a reasonable person can't work it out. It is reasonable they may have mixed feelings for awhile, but you will get what you want (a stop to that behavior) most of the time. It just may not be as comfortable as you'd like it to be Which is okay.
Similarly, someone who responds to a request of "Please don't joke on the ooc channel about PKing my pc or If you have a question or a concern about what I'm doing with this job/requestplot ect I'd like it to be brought to me privately rather than you questioning me about it on pub channel as soon as I log in" or even worse "I really love the relationship we've built between our two PCs but I would like to branch out a bit with our play and not TS/do slice of life/whatever with our PCs for awhile, can we please do X or Y or Z instead to take a little break?" with "ZOMG WELL SINCE I'M SO HORRIBLE I'LL NEVER RP WITH YOU AGAIN AND YOU WILL NEVER HAVE TO DEAL WITH ME EVER AGAIN!!!!" is clearly not someone with healthy boundaries or probably even capable of discussing that with you. You're going to have to be uncomfortable, but probably they'll no longer be pestering you or making public comments about you. You cannot control them or their feelings. Putting limits to how much of that you will tolerate before speaking up or asking for intervention won't control it either. Nor will it absolve you of discomfort.
Knowing other people don't like you, or that you have inadvertently hurt someone else--is not comfortable. Sometimes it's helpful to have a plan on how you will act/what you will and won't give should your own behavior be brought to light as having hurt or made someone else uncomfortable.
Boundaries just are not magical things. I think they're most valuable for keeping your choices and your own behavior on the up and up.