Created attachment 360182 [details, diff] patch for rxvt-unicode-9.18.ebuild rxvt-unicode-9.18.ebuild call epatch_user only when USE=-vanilla, but it makes sense to call it in any case. Patch for ebuild attached.
Use flag vanilla was dropped in 9.22-r1 bug #613724. https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=c9aee86fef439a3ead40700ff2fb120d19b44bc6