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 “Beliefs Clarified,” 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, Lord’s 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
