I'm looking at you, repost stream

Imgflip adheres to the Digital Millennium Copyright Act, it's in the Terms.

I think some blank meme templates have been taken down from DMCA requests.

Not official legal advice, nor Imgflip's legal stance but I would read this:
https://natlawreview.com/article/who-owns-memes

That article is what prompted my initial question. It appears memes are subject to the same fair-use policies as any other copyrighted work, even if the fair-use consists of publishing them as is. As usual the factor tipping the scales towards fair-use appears to be monetization. No point in suing without reasonable expectation of a payday. Good news for free speech. Also the only sensible way to handle something that by definition involves copying other people.