That's not really true. Many websites version their API and/or release new features without providing support (at least initially) via the API.
That's the route Reddit has taken. There are several features that only work through the official app or website. It can be frustrating as a user of a third party client but it's a much better alternative to cutting everybody off.
What I said is definitely true - supporting that old version isn’t free - it needs to be maintained and all new features need to not inadvertently break it. I’m not saying this was necessarily a good move, but the upside to killing an API is you are able to cut any need to support any of it, including old versions.
With feature iteration at Reddit accelerating since 2019, they may opt to do the same eventually should a desperate squeeze of user metrics/ad revenue becomes necessary down the road. Public APIs helped Reddit rapidly grow its userbase on a lean crew. It'd be a shame to see that goodwill being burned in the never-ending chase for quarterly performance results.
How many people use third-party Reddit clients versus using third-party Twitter clients as a percentage of total users on each site? (I don't know—not a rhetorical)
3rd party clients were the only usable mobile clients for quite a while. Reddit acquired one of them and has been pushing it but AFAICT as a Reddit Sync user nothing has changed. I imagine they are still pretty popular.
That's the route Reddit has taken. There are several features that only work through the official app or website. It can be frustrating as a user of a third party client but it's a much better alternative to cutting everybody off.