ohhhh this is a good question and i'm having a hard time pinning it down into a simple answer. i think generally speaking the big determinate is that it's a relationship where one or both of them causes the other some form of harm and/or the other person and the relationship becomes detrimental to that person... so it's not necessarily about specific dynamics or actions, though that factors in, but a bigger part of it is their effect on each other.

and while i think abuse can fall under toxic in the general sense, there has to be some kind of reciprocal interaction/effect to make a specific ship "toxic" instead of Only Strictly Abuse, i guess? as i'm writing this i realize i sound like "ABUSE ISN'T TOXIC YURI" antis on twitter, but obviously i don't object to them being grouped together. what i mean is when i'm looking at specific ships, "toxic" and "abusive" have different meanings to me lol, even if they can overlap in general convos.

WHICH IS STILL VERY VAGUE so for some examples.

like to me shymurr from mahoyaku are insanely toxic. shylock is literally consuming parts of murr's soul to keep shaping him how he wants/keep him attainable to him, but also murr is aware of shylock's feelings shylock's obsession with a man who can't love him back is not good for him either. they're crazy... i love it.

futakao from revstar with my toxic queen kaoruko. she's manipulative (to the point of pretending to sacrifice or harm herself) to purposely make futaba give up things to her! but futaba can't quit her.

meanwhile i think you can include pre-Big Reveal arlefuri in the "toxic yuri" umbrella, but i'd consider that one closer to specifically abusive/dub-con because furina is just terrified the whole time.

i could probably think of more but i'm getting distracted so... that's how i think of it!