I recall education Blacks in the 1960s using this pronunciation. It seems that when Blacks used this pronunciation back then it signaled an educated person. I just went back to recorded speeches of Shirley Chisholm. She used this pronunciation. Then from usage by education blacks that pronunciation seems to have entered into mainstream Black pronunciation and from there into common, widespread, mainshtream White usage. It's as fingernails on a blackboard for me. It's now 2016 and I'm noticing that the "h" is now being added to "st" strings as in "shtory". Anyone else follow this thread from educated Black speech? I know language changes. It's a natural process but my ear catches on every single pronunciation of "st" and "str" as "sht" and "shtr". The silliest pronunciation recently heard was largest trees pronounced as "largesht shtreesh". Totally mangled in other words and barely intelligible.