The things New Horizons Items which make my Reddit feed and top the list on sexy following the sticky are varied on subject so I would not say there's an issue with shared occurrences that community upvoting and downvoting does not already solve.
I usually think spoiler tags are somewhat absurd, but you'd want to go further than just time travel in the event that you really wanted to prevent spoiling things people have not seen yet. All out of current season content will require a tag weather it's an old image from the image library or even a new image from time traveling both will show new gamers thing they haven't already seen. Plus things like the latest direct video clips would need tagging since it reveals new content before anybody gets a chance to see it in-game.
I really don't see any problem with memorials. I think remembering your loved ones is beautiful and shouldn't be judged just as something dark or frightening.
I really don't think spoiler tags for content that has been in the game since launch is fair. When the match is out its out. Anything found in the game after that's fair for viewing. So far as New Horizons go, how can spoilers for content even work when there are two hemispheres and several time zones? When things and events become naturally offered in a special timezone beyond the US, if person be downvoted for what's rightfully available to them? Northern players for example spoiled things all year for southern hemisphere players rather than a word was said about it. Southern hemisphere players were admonished all summer since we obviously had snow. Yet few complained about all of the summer things that northern hemisphere Cheap Animal Crossing Items could show off. What's that fair?