The sdcc binary currently calls a recent (12.1) own version of cc1 which was traditionally the real challenge to build for older systems. E.g.:

Red Hat Developer Toolset 12.1 exists for RHEL 7.9, but CentOS 7 is EOL since June 2024:
https://www.redhat.com/en/topics/linux/centos-linux-eol

Producing static builds for Linux would probably be too hard at least as long as the recent cc1 is needed. And even without cc1 dependency the other versions of included GNU tools could potentially remain an issue, if the more recent versions are needed.