Well that took a sudden dark turn. Don't upset the Yandere I guess :D
Most sites I've seen don't really have a good system for content warnings. Something like a "Content Warnings on page 2" tag with a second page inserted that people like me, who generally don't want spoilery content warnings, can deliberately skip over, but are always available to those who do want them. Getting all uploaders to include them, let alone retroactively including them, would be hard I guess. Putting them into the first post of the thread (and reserving that post, which LubeTube does anyway) and adding a tag "content warnings in first forum post" would probably be an easier way to do it.
The latter option has been discussed previously on Dynasty, but I have no idea why nothing like this more elegant system (avoiding spoilers while still providing info on sensitive content) is in place, still.