Tldr They started charging 3rd party apps to use their API. The API that they had been using for ages and ages. They were shady about the timeline and their communication to devs was awful. So most 3rd party apps decided to end support as opposed to paying reddit.

Basically they wanted to make sure all users were seeing ads before their IPO.

I was a relay for reddit guy for ~10 years. Once the subscription update hit I uninstalled and moved to boost for lemmy.

Mods used third party tools to help them with their unpaid work. These tools relied on access to the reddit api.

Then reddit charged ridiculous prices for api access, which would mean the apps would need money, so mods would need to pay to do their unpaid work or use the inferior reddit interface.

Given these choices, some mods decided to leave instead.

Reddit used to have a free and open API. This allowed 3rd parties to develop apps / interfaces for the site. These apps helped everything from making the site usable for some with accessibility issues to blocking ads to providing a customized interface to tons of other things.

Generally this was done by taking the API feed and re-engineering it to allow the desired presentation.

In a move to make the company more attractive to investors before going public, Reddit changed that API to a paid model. This meant any developer of those 3rd party apps would now have to pay a not insignificant in most cases fee to continue their access to Reddit. As such, most apps closed down and a very small portion of us long time Redditors migrated to Lemmy/ the fediverse.

Reddit used to have an open API. A lot of mobile apps sprung up to access reddit over the years, with different features. Reddit gained a lot of loyal members through users of these apps, but couldn't make ad revenue off them. Reddit decided last summer to start charging a lot of money to these app developers to continue using the API. A few of the apps started a for-pay subscription model to continue operating, but many just shut down their apps. Many redditors and Reddit mods revolted, because these apps made the site usable (some of them offered advanced mod tools, etc). We protested, shut down subreddits temporarily or permanently, deleted our accounts, moved to new platforms (like lemmy/kbin), etc. This was basically a move to maximize their ad revenue while Reddit positions itself for an IPO. It was really not cool.

There was some magic internet data thing where third party apps (like various reddit apps not owned by reddit) access and use reddit to power their apps, free of charge. That's API. Pretty much all voluntary reddit mods use third party apps to mod. Reddit started charging for the API, making third party apps useless, forcing everyone into the Official Reddit App. A lot of people boycotted or quit.

It was a thing last... summer? Where reddit announced they were going to start charging for every call a third party program made using the API. This was done with the intent of shutting down 3rd party reddit apps and to get users on the official one so reddit could make more money. However, it also destroyed a bunch of 3rd party tools that mods more or less needed, and which reddit had been promising to implement themselves for years with no progress. There was a brief protest where mods of many subs shut them down (mostly for less than a week, though some are still down IIRC). A bunch of users and moderators left reddit and went to other sites.

Others have covered it but the API pricing was $12,000/50 million requests, which is absurd bordering on comical.