I do remember the “download a big movie and then delete it” hack. Didn’t think that would still be useful.
Given all that, I can see Apple not fixing something like this because it might break all sorts of other stuff.
Apple had one minor update where the caches did not auto-clear several years ago. I suspect that is where this claim comes from. Apple got a lot of complaints from users very quickly as RAM started filling up with junk and Apple issued an update within about two weeks that fixed the issue that came from an earlier fix that caused the problem in the fist place. Apple users are VERY VOCAL about such issues. They do not allow such obvious problems to stay long!
Contrary to the myth, The data on iPhones can be seen by the users and the usage of the memory and caches is also easily cleared.