Is your feature request related to a problem? Please describe.
No, I think.

Describe the solution you'd like
The example mod for Legacy-Fabric seems similar to the example mod for the current Fabric version.

I can offer you the 1.8.9 forge branch from 5 years ago: https://github.com/TechReborn/TechReborn/tree/1.8.9

I dont think this is something we are going to do officialy as its 100s of hours of work. Feel free to do it yourself. (Id recomend taking the 1.16 branch if you do)