My personal use of email agents involves ordering incoming messages by date sent. Many others order their inbox by date received. I don’t use MUA ordering by thread or conversation because I must use MUAs on many diverse systems. So for many decades, I have continued to use my own mental programming to sort and group messages by subject. This, by the way, is akin to aural conversations between persons where announcing a change of subject is expected to be followed by a new topic.
For those of us on OCD, ADD, or Autism spectrums, multiple subjects lines cause cognitive dissonance - sometimes damaging comprehension of the continuing thread (conversation). I don’t claim it violates the ADA, but it should especially when willfully continued after requests for amended behavior. Lazarus Long would probably express this more cogently.
In the interest of polite conversation,