This was brought up in issue 849342. As Chrome evolves and features change, many preferences get disused. In some cases (such as with content settings), they may contain URLs or other kinds of PII.

Find a way to ensure that these preferences are properly cleaned up.

- In the client, we could introduce a single place in the code where disused preferences will be listed and a cleanup code will run. Developers may add deprecated preferences and get them automatically cleaned up (e.g. on startup, regularly, etc. - up to discussion).

- Serverside. The Sync server could do a cleanup of serverside sync preferences that are marked as deprecated, and stop syncing them in the future.