EDIT: some information in this first post is no longer up-to-date.
Please see the update further down. /EDIT

qlstats will become opt-in based. Players will have to register by using their steam login and accept the data privacy policy in order to have a match history and a rating. Unregistered players will show up in match reports as "Untracked Players" and no reference to any personal data (including steam-id) will be stored by qlstats.

However, since untracked players by definition can't have a rating, and the rating process for a match depends on every player having a rating, a match with an untracked player in it cannot be rated for anyone.
If a server owner want to run a "ranked server" where all matches get rated, he will have to use a minqlx plugin to ensure only registered players can join the match. This is not because qlstats is evil, it is a requirement for the system to work. Ratings don't appear out of thin air and matchmaking depends on player ratings.

A player will also have the option to delete his account at any time. qlstats will keep only the steam-id and the date of deletion on record for 2 months to prevent re-signup, everything else is deleted immediately and in the existing matches the player is replaced with an anonymous placeholder.

If a player does not register until May 25th, he will be treated as "not tracked", but the data is kept for 2 more months to allow the player to register or delete it explicitly. During this time the player's information will not be accessible via web site or API. After those 2 months, data of unregistered players will be deleted and matches anonymized.

This is just a heads-up, more updates will come throughout the next days.