Custom tags are a powerful feature. for reasons I'm confident you're aware of.
However, once you create a custom tag field, say "PM_Ideas", there is not the ability to edit or delete the tag value.
This is very limiting in such key ways as 1) Once an item has been decided to be merged into another tag category the old tag definition still show up on the list, and 2) When you want others to use tags for that fields, you will find the too many similar tags with no way to consolidate from the list which is displayed.
For the same reasons the default tag category can allow edits, deletes, renames so should the custom tag fields.
|Release time frame|