For the uninitiated, generally NSFW is for sexual contents and NSFL is for gory contents. People may want to see one but not see the other at any time for any reason. I have seen this feature requested over the years in Reddit but it never happens. Maybe now some instance can finally implement it?
Intensity warning is a good thing. Though it does make tagging complicated, but in this case overlapping tags would do. blood-death and gore-death and simply death.
probably shouldn't borrow the exact terms from fandom, but they have tag modifiers like 'dead dove: do not eat' which basically means this is an absolute celebration of the previous tag, so gore tag coupled with that tag is gore intensified to the max, while they also use tags 'slight mentions of gore' for only a bit of gore. but if you filter out gore both would still be filtered out.
AO3 runs on open source software and has a very robust tagging system.