you can first download the LuCI 15.05(???) sources and the needed corresponding OpenWrt main sources, then you can identify the necessary commits for the token removal, and backport those commits to 15.05, compile LuCI, install & test it and be happy. (based on the on following search, the relevant timeframe is October 2015. https://github.com/openwrt/luci/search?o=desc&q=token&s=committer-date&type=Commits Note that not all relevant commits are shown here, as that only search for commits with "token")
In practice, no chance.
Token removal required 30-40 commits from the main developer, so unless you are LuCI specialist, no real change to easily backport them correctly.
Older versions are not secure. Security is worth the time of upgrading to the latest supported version. Developers don't just update the firmware because they are bored.