Fun things is that I encountered that for the first time when using Clozure CL (https://ccl.clozure.com/) which quotes colons when converting paths to string even on Linux:

  $ cat <<'EOF' >x.lisp
  heredoc> (require :uiop)
  heredoc> (let ((p (make-pathname :name "foo:bar")))
  heredoc>   (format t "~@{~A~%~}" (namestring p) (uiop:native-namestring p)))
  heredoc> EOF
  $ ccl -b -Q -l x.lisp </dev/null
  foo\:bar
  foo:bar
  $ sbcl --script x.lisp
  foo:bar
  foo:bar