The longest extant, human-made palindrome, I believe, is this one, as featured on palindromelist.net and copied elsewhere throughout the internet throughout the 2000s. Notably, this appears to be posted without credit, and finding the credit for it was a pleasant challenge. This one had been kicking around in my head for a long time ever since I first came across it as an 11-year old. I misremembered the opening line as "A star? Not I!", and when I searched for it, I found a Reddit comment that seemed down on it:
Yeah, the claim is silly. I can just put some more words before and after it, and it will only be slightly more incoherent. A man, a plan, a Star? Not I! Movie – it too has a star in or a cameo who wore mask – cast are livewires ..[tons of nonsense here].. dosser I – we – vile rat, sack! Same row, oh woe! Macaroni, rats, as a hoot, tie. I vomit on rats, anal, panama. There, now i have the longest palindrome.
First of all, I highly doubt that this great palindrome is nonsense; at least one other commenter in the thread has found meaning in it. Secondly, I had first assumed "Anal Panama" to be a ripoff of xkcd's comic about palindromes, as featured at the top. Turns out, however, this comment predates the comic by nearly two years! Watch this space for more details about u/ABCosmos' pending lawsuit against Mr. Munroe. But this doesn't answer where exactly this palindrome came from.
Kudos to this surprisingly enlightening comment thread on Grand Text Auto, a seemingly-defunct blog on video games hosted by UC Santa Cruz, for having the most information, and for ending the trail of finding credit for this wonderful palindrome.