So here's what finally went in:
- The stock UI still uses character name. It will migrate settings tables between character names if the name changes. This has the least chance to break addons that are depending on the structure of the stock saved vars, is easiest to read, and is safe because the stock UI won't be disabled like an addon.
- We re-added the character ID constructor in ZO_SavedVars named NewCharacterIdSettings. Addons can migrate to character ID settings for a solution that is robust against name changes when the addon is disabled. Character name based settings will be auto migrated to character ID keys the first time the addon is run using NewCharacterIdSettings.