I propose increasing the lowest supported Serlvet/JSP versions to 3.0/2.2, starting with the next release (FreeMarker 2.3.33). From Tomcat perspective, that's Tomcat 7 (2011-01-14, EOL since 2021-03-31). This will simplify the source code and the build a bit, which will also come handy for generating the Jakarta version.
Currently (2.3.32) the lowest version we support is Serlvet 2.4 and JSP 2.0, and then there's some special support class for JSP 2.1 (that's a complication we could drop with this). With Tomcat versions: 2.4/2.0 is Tomcat 5 (2003-12-03), and 2.5/2.1 is 6 (2007-02-2, EOL 2016-12-31). I think somebody who still uses such ancient servlet containers is unlikely to care about updating FreeMarker. Any thoughts?