Although developers referenced it, one of the most incredible things about Steam Spy (read: a now-defunct third party public-facing data aggregate) was how useful it was as a consumer. If a developer wanted to lie and say their game was selling "amazingly" and that "everyone should get in on a huge community" they could do it -- nothing is stopping them from hyping something up on empty promises.
But with Steam Spy you could, through at least some sort of metric, and say "hey, maybe this game isn't selling like hotcakes," and you'd know if the matchmaking system would be dead in a few weeks (case and point: Battleborn). But that's gone now, as Valve effectively altered their privacy settings to eliminate that tool. Now they're working on tools that will help developers see more accurate metrics.
Speaking to an indie developer at the White Night developer event in St. Petersburg Russia, Valve's Jan-Peter Ewert explained that the company is working on a system that's "more accurate" than Steam Spy to assist developers. Steam re-iterates that they dumped Steam Spy due to a "lack of compliance with GDPR," and because "it was very accurate for some games, it was very inaccurate for a few others."
Again, power to the developers for getting tools to help them -- but now consumers are once again in the dark (what else is new though when publishers don't share digital download data and Microsoft isn't even sharing hardware sales?).
Valve plans to replace Steam Spy with “something better” [PCGamesN]