The index holds the values previously calculated in coinstats.cpp for each block, representing the state of the UTXO set at each height.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/19521 | core#19521]] [4 & 8/17]
https://github.com/bitcoin/bitcoin/pull/19521/commits/dd58a4de21469d6d848ae309edc47f558628221d
https://github.com/bitcoin/bitcoin/pull/19521/commits/57a026c30fef3138bb8db46e6865acb9dc2674f8