@ummeegge git repo has version 1.20 from 2018.
The last version that was in IPFire was 1.22c from 2021
The current latest version in the sslh github repo is 2.1.2 from May 2024.
It seems that release versions are occurring more frequently now from every 1 to 4 months.
If you are looking at building sslh then get the latest version from the github repo
https://github.com/yrutschle/sslh/tags
and use the lfs from the last ipfire version as your starting point.
The rootfile and initscript are also in the repo but they will need some modification. The rootfile from 2.1.2 is likely to be different from 1.22c and the initscript from 1.22c already did not work due to changes in the configuration parameters from version 1.7a to 1.22c
In this post another user indicated that the changes required were relatively simple so you could use those as a starting point.
https://community.ipfire.org/t/sslh-add-on-not-starting/11413/4