On many sexual posts in the subreddit there is an nsfw tag which can be useful to those who wish to filter out such content. But often on questions that discuss issues such as rape, torture, etc. there is no content warning of any kind. Would it be possible for such a tagging system to be implemented - especially for those readers who suffer from PTSD.
We actually do tag much of that NSFW automatically. It is done by keyword though using Automod code, so there are some phrasings that don't use the usual terms we expect (or words that we don't include in the filters due to too high a rate of false positives), so some things do get through untagged. If you see anything that you feel ought to be and got missed, please do report with a custom message requesting NSFW marker be applied. Thanks!
I think AskHistorians already automatically tags rape questions NSFW. Did you have any specific examples you could point to?