Well, searching for "new content" gives me the following, in this forum (Bug/Features) over the last ~6 months:
Bug - Fixed missing reanimated skeleton info
Bug - Fixed The travelling trader trades Tacos (Fluxxdog reiterated the need for a 'New Content' tag to differentiate from bugs in this thread)
Bug Level 6 redone (Fluxxdog mentioned the need for a 'New Content' tag to differentiate from bugs in this thread)
All 3 were regarding new content being added to the game, only 1 of the 3 was from me, all 3 were actually implemented, and in none of those three was the tag questioned (though Fluxxdog mentioned that the new tag would make less of a shock on the 2 older tags)...
Regarding you asking why in any reason, in threads marked 'bug' over the last ~6 months, I found the following:
Bug Dusty Bottles automatically made a goal (You asked why the goals being set was a bug, not related to new content)
Admittedly, the post searcher only displays the first three lines, and "why" can't be searched for as it's too common, but... that's the limits of my searching abilities. The standard from the searching with the posts above and the board in general appears to be to tag new content with bug currently, since it generates exceptions to the mafia data files, causing overrides and large groups of people posting bug reports 3-4 months later when people's scripts that involve new IotMs and the like are generating errors because they need to update clear again...
However, as I said, if you prefer me to use FReq in the future on it, I can. It just doesn't seem to have been the 'standard' tag for it. In my admittedly limited experience. We're hampered in knowing which you/Jason prefer by that, when you have new content, you generally just implement it, you don't need to post that it exists.