I'm also unsure how much the exclusion of mixer campaigns has reduced spam, as those campaigns primarily featured posters that were generally considered higher quality.
I am not sure about posters in mixer campaigns being considered generally higher in quality or whether spam has been reduced ever since mixers were banned. What I do know is that it seems to have become easy to spot some accounts that are obviously part of a farm. It might be because the campaign space is limited to participants after the void left by mixers.
There is another reason for theymos to not rethink this over:
Blender operated from approximately 2018 to 2022. The service was advertised on a popular internet forum as having a “No Logs Policy” and deleting any traces of user transactions. ~
If mixers return it might put the forum back in the spotlight for the wrong reasons and for that reason theymos will probably never rethink it.