Not yet, someone requested this feature and I would like it to to block the NSFW porn instances. I do have NSFW enabled but I still see pictures of SFW girls in my feed from NSFW instances and I would like to just block that stuff from those adult only instances.
I not for lemmy.world blocking or deferating instances in general. But this feature gives the user the choice to block instances while others can allow content from instsnces to flow into their feed if they wish.
I only use FOSS apps. I am sure someday it will be added into Lemmy itself.