"Deprecate" rather than "delete" entries

[swickr]

The 23 July Working Draft proposes that "delete" is an allowable registry operation. I encourage the WG to consider whether best practice might better be to mark an entry as "obsolete" or "deprecated".
This preserves the history of what it had been when someone is trying to understand old code and to avoid harmful reuse of an identifier, as noted in #5

"Delete" might still be permitted but reserved for rare use.