Here's the announcement post. Happened last month.
The gist is they want to be able to charge anyone pulling posts from the API (like AI training bots, third-party Reddit reader apps, etc) a fee and also stop third party apps from being able to show NSFW posts and subs. The announcement post has been downvoted to hell but because of Reddit's new shitty feature of not showing negative downvotes on posts it only shows as having 0 votes. Everyone in the thread hates that Reddit has done this even the asshole mods complained. Didn't matter, because Reddit is anti-user.
The change has crippled sites like Reveddit that used PushShift api aggregation to recover posts and comments censored by mods/admins.
Edited 11 months ago
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Time to start scraping then
A group called The Coalition for Independent Technology Research has started a campaign to try and fight the API change. Knowing Reddit nothing will come of it however if anyone cares here's their post about it: Reddit Has Cut off Historical Data Access. Help us Document the Impact.