The WTS did use the Studies in the Scriptures up till the Harp of God came out in 1921. Any copies of the Studies in the Scriptures were cleared out of their stock much like the do out of date publications today. I have heard that some were burned or trashed but I have not found any confirmation on that in the WT publications.
w55 12/15 p. 755 Blessed Results from Making the Test ***
The old Bible textbooks, the seven volumes of Studies in the Scriptures (1886-1917), were replaced, for, beginning with The Harp of God of 1921, a new series of Bible study textbooks was published culminating in the book You May Survive Armageddon into God’s New World in 1955.
w54 6/1 p. 344 1926 Ushers In a Time of Great Happiness ***
Prior to 1926 Jehovah’s people had very little in the way of equipment to work with. The seven volumes of Studies in the Scriptures had served their purpose and were going out of stock.
w93 11/1 p. 12 Daniel’s Prophetic Days and Our Faith ***
At the same convention, the hard-hitting book Deliverance was released, becoming the first of a series to replace Studies in the Scriptures.
*** Daniel Prophecy book chap. 17 p. 304 Identifying True Worshipers in the Time of the End ***
At the convention in May 1926, the book Deliverance was released. (See page 302.) This was one of a series of new books designed to replace Studies in the Scriptures. No longer were the holy ones
w67 1/15 p. 54 The Modern Fulfillment of the "Penny" ***
But in 1927 The Finished Mystery and the other six volumes of the Studies in the Scriptures ceased to be published, and they were replaced by new, postwar Bible-study aids.
*** Kingdom of a thousand years book (1973) chap. 17 p. 347 The "Slave" Who Lived to See the "Sign" ***
Later in the year 1927 any remaining stocks of the six volumes of Studies in the Scriptures by Russell and of The Finished Mystery were disposed of among the public.
Now, why the Watchtower would sell these books, which they felt
contained false teachings and tended to "cause a sect centered
around a man" to the public at all, is a seperate question which
will not be addressed here. The issue is, are the statements in
the 1973 book honest, about 'disposing' of all remaining stocks of
these false books in 1927? Apparently not. In 1929, that same set
of books was being pushed in the door-to-door work; the week of
November 10th was designated a "special drive week" for them.
In 1932, the bulk of the books spread by the Witnesses was again
that same set. In 1944, they *still* were to be found on the list
of books offered. And in a 1967 "Kingdom Ministry" (a monthly
magazine handed out to Witnesses, with internal notices, etc.)
they were listed as "out of stock" in the USA.
In the "Watchtower Cost List" of February 1, 1962, on page 12, under "Other Items" there is listed, the set of Studies in the Scriptures. 35 years after 1927, as late as 1976 in the Society's "Kingdom Ministry" there is an announcement that "The Battle of Armageddon" Volume 4 of Studies in the Scriptures, was now out of stock. So, were the researchers of the "God's Kingdom" book, careless or were they trying to hide the truth? No one can know for sure, but there have been other discrepancies in the Society's history.