This has been part of Tomcat for some time it seems. I was able to compile and merge tomcat 8 without. I believe the same for older versions of tomcat. That needs to be tested and confirmed, as I have not tested Tomcat 6 or 7. I don't care about the older versions as I am using 8.
09 Jun 2015; Patrice Clement <monsieurp@gentoo.org> +tomcat-7.0.59-r1.ebuild, +tomcat-8.0.22-r1.ebuild, +tomcat-8.0.23-r1.ebuild: Remove jakarta-jstl dependency. Fix bug 551032.