> Since when can memory leaks only be observed from within 
  > the language itself?

The issue you linked has to do with leaking local variables during a panic, and if uncaught a panic will bring down the entire process. I don't see how you could observe such a leak.

  > Additionally, the entire issue brought up in the 
  > deleted comment was in relation to threads, an argument 
  > of a lack of user-defined destructors wasn't brought 
  > into question at all.

As party to the original comment, it didn't bring up threads at all. Nor does the issue you linked above, #14875, relate to threads at all. To paraphrase what I remember of the original comment, it was "I stopped using Rust when I got to the Dining Philosophers chapter of the book and found that it taught newcomers to leak memory like crazy, via a bug that has gone unfixed since 2014." It contained no mention of which bug that was nor where the leaks were, hence the numerous follow-up comments asking for clarification.

  >  The original comment mentioning this issue was lacking 
  > so in confrontational qualities that the responses he 
  > or she received here and on twitter (how I myself 
  > arrived here) were entirely unwarranted.

I can only guess that the original poster edited their comment between the time that you and I read the post, because the original version was quite confrontational. What responses do you believe are unwarranted? All the ones here seem courteous, and I haven't seen any on Twitter (I would appreciate it if you could link me to these).