Attackers could otherwise verify that resources on private networks or retrievable only with the victim's cookies have certain content.
See, e.g., <https://github.com/w3c/webappsec/issues/418> for some broader discussion.
See, e.g., <https://github.com/w3c/webappsec/issues/418> for some broader discussion.
The possible solutions to this punctuation-following-URL problem are that you delimit the URL, contort your sentence so the URL is followed by a space and some other words instead of punctuation, start adding random whitespace after the URL but before the punctuation to avoid the linkifier eating the punctuation, or stop putting URLs in plaintext. I've seen all of these used; the first solution is by far the best.
Oh, and that's all from a Western perspective. If, on the other hand, you're using a language that does not use space-separated words (e.g. a number of East Asian languages), then delimiting becomes even more important, because you can't just guess that the URL ends at the space character; there are no space characters around.
I can't speak to your experience seeing or not seeing this syntax, but as I said it's been part of the URL RFCs for over two decades, is used in other RFCs where URLs can appear (e.g. the Link header syntax), and is reasonably commonly used by people who both put URLs in their email and want to punctuate it properly. I will grant that proper punctuation is out of fashion in certain demographic groups. As is writing plaintext, I guess.