"Extreme content " flags on writing are generally reserved for anything of graphic nature, be it sexual, violence, descriptive gore, etc.--basically anything that you (in general "you", not just you, personally ) would not recommend that a child or anyone of the age which requires parental consent, to be reading. Also those who may be faint of heart, regardless of age.
We do have such a population of people who visit this site, and while we cannot control who is able to read, what, extreme content is our way of covering legal aspects that may apply to certain readers .
If a piece of writing is such that it really wouldn't be a big issue if a child or very sensitive person were to read it, then it really doesn't need a content warning. There will invariably be differing opinions on what is interpreted as "extreme", but if you run it through the filter of: will this give someone nightmares, or is this X-rated, or do I want my 12 year old, reading this, etc? Then that is the standard by which flagging or not flagging poems is determined.
Some readers do indeed, avoid the writes that are flagged extreme, so you could actually be losing a reader-base by having something notated as such, that really isn't in line with the aforementioned examples of what should be flagged.