I think part of the issue is how exactly does one define toxic?
Toxic to the household?
Or toxic to the new partner?
Who identifies what toxic is?
My husband identified himself as toxic to the family as a whole (including my other partner) and moved out.
When he had a potential who tried to identify me as toxic (because I wouldn't allow her to decide when and where my kids joined in her activities & wouldn't allow for her to walk all over me in my home)-
the whole family told her to fuck off. Shrug.
I think the issue is that metamours really can't make these decisions. Roommates maybe.
A metamour could decide it was toxic for THEM and remove themselves from the situation. But unless you are part of a relationship with someone it's not your place to decide what to do within the realms of that relationship.
In our home-Maca and GG are metamours-but they are also coparents and roommates. As roommates and coparents they can address issues with one another. But neither would deign to comment on the relationship the other has with me. It's not their business.
If either has an issue with something in our relationship-they can renegotiate with me over the details of how we run our dyad. They can TALK to each other about it-but it's not their dyad so it won't do any good.
The same is true with outside of the home relationships. I refused to participate socially with 20s girl. But I didn't tell Maca he couldn't. She was toxic-HE decided she was toxic for him ultimately. But it wasn't my place to make him see that. I saw she was toxic for me and for our children. It was my job to ensure that she was away from us. (we all agreed she was toxic for the kids and we all agreed she couldn't be around them-but it was each of our responsibilities to assess that as coparents anyway).
The best thing to do with a metamour (or any other person) who is toxic for you-is not to participate in socializing with them or hearing about their drama. Ever.
"Love As Thou Wilt"