Can you imagine if some sect of NFL fans demanded the new commissioner prove they were around before instant replay?
I'm sure many older Oriental Adventures makes sense. But I can see why some folks would consider it unfair to suggest that new fans should be equally cautious when perusing, say,
The creators who worked on products that weren't really problematic, but are having their works lumped in with material like Oriental Adventures, for one. Some of whom still work on the game today and are clearly working to make it better, like Jeremy Crawford. (Unless he's disavowed Heroes of the Feywild as an old shame and I missed it?)
And the fans who enjoyed said products (to include fans who jumped on with 4E just before the 5E switchover), and could read in that disclaimer the implication that the things they liked were wrong, whether or not that implication has merit in an individual case. Many take such things as implicit criticism of themselves as well as of the work.
Note, arguing that folks shouldn't be offended by something is generally a non-starter... all you can do is try to understand and accept why they might be offended.
I can't think of any real life person that would be offended by this stuff and my friends and I harken back ADnD. I thought it was the snowflakey hypersensitivity of the newer generations that was derided by some of the older players. And now some of them would get offended by a warning or disclaimer? I don't think they care. Just like the clown joke. It's probably only a few loud people that are actually upset by it. If they are, just boycott and don't whine about not getting catered too.