I'm surprised this new(-ish) feature hasn't gotten more traction
Let's review....
The OP refers to the Watchtower 15 February 2017, page 26, paragraph 12 - from a study article that most congregations will be studying the weekend of 29/30 April

Note near the start: "the Watch Tower Publications Index includes the heading āBeliefs Clariļ¬ed,ā which lists adjustments in our Scriptural understanding since 1870."
The OP then linked to the WOL version of this (the 1986-2015 Index) at: http://wol.jw.org/en/wol/d/r1/lp-e/1200277174
The 2001 - 2015 Index (you need pages 46-47) is available to download as a printed PDF version at: https://download-a.akamaihd.net/files/media_books/59/dx01-15_E.pdf
Now this is when it starts to get interesting, or maybe not....
The 'Beliefs Clarified' heading does not seem to appear in any earlier print Indexes - BUT it does appear as a heading in the CD-ROM version of the both the 1930-1985 and 1986-2015 Indexes - ie the heading has been added
There is downside - the Indexes only index the information published in the years they are indexing although that information is about earlier years
ie the 1986-2015 Index will only make references to publications in those years
So, for the year 1872 the Indexes reference
Index 1986-2013
1872, ransom: jv 718
Index 1930-1985
1872, Lordās return: jp 15; w34 11
As the 1986-2015 is available online at http://wol.jw.org/en/wol/d/r1/lp-e/1200277174
FYI, below is a screen shot from the CD-ROM of the 1930-1985 Index
BTW, this has also been partly discussed previously here https://www.jehovahs-witness.com/topic/273008/new-light-beliefs-clarified
