In #14822, we introduced the ability to guard against the use of the "reserved" field name tags. As part of that change, we introduced a setting event_api.tags.illegal that could be set with warn, which would still allow the previous behaviour, and continue to set the field tag, and only warn against it's use, and rename, which would move the illegal value and put it into a new _tags field.

We should remove this setting, and always use the behaviour currently provided by the rename setting

• Mark setting as obsolete in 9.0
• Update 8.x deprecation log to state that this setting is being removed in 9.0