[meta] SSS & PPS

Nobody has claimed this yet.

_Here be dragons._

source

(that's a draft)

This is a meta issue to track the work about _Simplified Sliding Sync_ (SSS) and _Pragmatic Pragmatic Sync_ (PPS)

# Simplified Sliding Sync

Synapse is implementing Sliding Sync natively. Since [the Proxy implementation](https://github.com/matrix-org/sliding-sync) was experimental and was exposing too many features we now know we don't need, we call this project Simplified Sliding Sync.

## Tasks

- [x] Replace the sync `ops` by client-side sorting
- [x] #3068
- [x] #3585
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3664
- [x] Ruma support
- [x] https://github.com/ruma/ruma/pull/1879
- [x] https://github.com/ruma/ruma/pull/1907
- [x] https://github.com/ruma/ruma/pull/1911
- [x] Move the entire SDK to SSS, and be able to switch to SS at compile-time or runtime
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3676
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3702
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3724
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3820
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3830
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3832
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3833
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3853
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3871
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3874

# Pragmatic Pragmatic Sync

Pragmatic Pragmatic Sync is another effort to extract some features of Sliding Sync into their own HTTP endpoints.

## Tasks

- [ ] Replace Sliding Sync by `/sync/e2ee`
- [ ] Replace Sliding Sync by the improved `/context`

# On the server-side

They are things missing or not working correctly on the server-side. We have nothing to do about it in the SDK, but it's important to list them so that we have a complete view of the project:

- **[Board](https://github.com/orgs/element-hq/projects/126/views/1)**
- [x] https://github.com/element-hq/element-x-ios/issues/3106
- [x] https://github.com/element-hq/synapse/issues/17540

# Release and Clean Up

Now it's time to clean up the old `matrix_sdk::sliding_sync` implementation.

# Auto select SS or SSS

- [x] https://github.com/element-hq/element-meta/issues/2499
- [x] https://github.com/element-hq/synapse/pull/17571
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3889
- [x] https://github.com/matrix-org/complement-crypto/pull/128
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/3908

## Tasks

- [x] Remove old features of Sliding Sync
- [x] Remove potential intermediate feature flags
- [x] Remove the `experimental-sliding-sync` feature flag:
- [x] https://github.com/matrix-org/matrix-rust-sdk/pull/4483
- [ ] https://github.com/matrix-org/matrix-rust-sdk/issues/3698

Review the remaining unchecked tasks under Pragmatic Pragmatic Sync and Auto select SS or SSS, especially issue #3698. The issue is a meta tracker rather than a code-specific task; done means the listed work is completed and the corresponding checklist is updated.

Written by the indexing model from the issue text.