In this case, is recommended that URLs be preceeded with a prefix consisting of the characters "URL:"
It's not clear that this ever became common--most use in the wild seems to omit the "prefix" entirely.
At https://www.colbyrussell.com/LP/debut/plain.txt.htm, I adopt an alternative "concise" form that permits the "URL" atom to be omitted, while still allowing for the colon to be left in, for reasons explained in an annotation left there:
doesn't lead to interference with the HTML5 parsing algorithm [...] crucial for compatibility with mainstream browsers--the leading colon prevents the the bracketed plain text from being interpreted as HTML
The idea is that the form with the leading colon should be treated equivalent to the form that omits the prefix entirely, except it may be used in contexts sensitive to parsing constraints as described above.