Add warning that REST API is no longer supported

[iamjackg]

Proposed change

According to home-assistant/architecture#1074 (comment) the REST API has been superseded by the WebSocket API. There was no mention of this anywhere in the documentation, so I'm adding a warning to let people know.

Type of change

• Document existing features within Home Assistant
• Document new or changing features which there is an existing pull request elsewhere
• Spelling or grammatical corrections, or rewording for improved clarity
• Changes to the backend of this documentation
• Removed stale or deprecated documentation

Additional information

• This PR fixes or closes issue: fixes Weather get_forecast service returns 500 on 2023.9.0 core#99820
• Link to relevant existing code or pull request: Add boolean option to `calendar.get_events` to merge output architecture#1045

[Klikini]

This really needs to be merged so people don't rely on the REST API for new projects and then get burned...not that I would know anything about that! 🙃

In addition, I don't remember seeing any mention of this on the blog. Has there been a post about it?

[sebsebseb1982]

Deprecating the REST API is a really bad idea and will put a stop to projects based on cheap hardware (esp32, etc.). Calling the websocket API on such hardware is unnecessarily complicated.
Very disappointed with Home Assistant and this "surprise" depreciation (ping @emontnemery).
Thank you anyway @iamjackg for your PR.

[Klikini]

@sebsebseb1982 I learned today that getting states with the Websocket API is actually impossible on memory-limited platforms like those.

I thought there would be a command message to ask the server for the state of a single entity or list of entities, but no, you can only request the state of all entities. This array would take up more memory than embedded devices have, and iterating over an array of a thousand or more entities would take forever even if it fits.

I would really appreciate some justification from Home Assistant leadership as to why the REST API is being deprecated superseded by the WS API, and an opportunity for the community to provide feedback on that decision that will negatively impact it.

[sebsebseb1982]

Thank you @Klikini for this warning.
I would have been very upset if I had also wasted my time using the WS API without it being viable.

[allenporter]

The rest API is not deprecated, going away, or planned to go away etc.

[krkeegan]

I just ran into issue home-assistant/core#99820 and spent over an hour banging my head against the wall trying to figure out why it wasn't working.

That is frustrating to say the least.

I agree it seems like the REST API is EOL, will no longer have features added, or at least the get_forecasts will never be supported in REST.

Since at least one of those statements is true, can we at least get a more clear error message to help others who wind up here?

[bdraco]

closing this PR per #2150 (review)