(Full disclosure, there was a question on this same topic 2 years ago that went unanswered, but figured I’d give it a try.)
In data returned by the API, there are some fields which don’t appear to be used anymore. Looking at an example player’s /games page, the fields include:
black_player_rating
black_player_rank
white_player_rating
white_player_rank
For all 4 fields, the mode value is 0. The average rating is 490 – values appear to be distributed normally around that point. (The mean current Glicko rating is about 1500.)
Are these fields from the system used before Glicko was introduced in 2017?
I assume the same is true of the various fields in the player API. The ratings[overall] field is correct, but not:
rating
rating_blitz
rating_live
rating_correspondence
ranking
ranking_blitz
ranking_live
ranking_correspondence
Back in /games, even under “historical_ratings”, the “ranking” field is also from the old system. Actually, it reports the current value, not the historical value – it doesn’t change from game to game.