How to do a "mutual ignore"?

No, that won’t be happening. If hardcore block* (which has never historically been a forum software feature anyway) is a written in stone requirement, you should switch to different free open source software. Sorry!

That being said, I think an API based mutual ignore is a fine workaround for the moment.

* block, meaning “additionally, this person can no longer see my posts” on top of “I no longer wish to see this person’s posts”