Concerning Spoilers

Yes, I think what's being said here is that for the vast majority of series, time is irrelevant to spoilers. The only real exception would be where knowledge of a franchise saturates the general public (or in this case the average Danbooru user), as I mentioned with Star Wars above (which virtually everyone has seen, and has had 36 years to become common knowlege).

So for Danbooru's public it's a pretty sure thing that everyone here knows "Aeris dies", but then that was another super popular franchise that has had 16 years to saturate the general populace. Very few franchises reach that level of saturation such that it becomes difficult to find people who don't know the spoiler. For series that haven't yet likely been seen by a large proportion of our user-base (most series), no amount of time will cause a spoiler's knowledge to saturate.

For example, even Puella Magi Madoka Magica - which was probably one of the most popular series during its run, and one which has had almost 2 years to percolate - I would still say it's very likely that a sizable proportion of Danbooru's audience would still be adversely affected by spoilers within, and they would ruin the series for people who hadn't yet seen it.