In this comment, @vandeberg confirmed that the next release will contain this feature.
Now, after enabling content editing with enable_content_editing json will I be able to edit my posts older than 7 days by broadcasting the transaction to your node(appbasetest.timcliff.com)? Or do I have to wait till all the witnesses update their nodes to the new version?
It will need to be running on witness nodes. You will not need all witnesses to update though. As long as the transaction is processed by a witness that accepts it - it will make it in.
Thank you for clarifying.
I see that the witnesses are still running on 0.19.2(at least top 100). Any idea on when are witnesses planning to update to the latest version?
19.4 is still a release candidate (not an official release). It is unlikely that any of the witnesses will run the non-release version in production, so at least until the official release is made.