This mod reduces the memory usage of Minecraft in a few different ways. A high-level technical description of the changes is available here. I am working on getting at least some of these changes added to Forge, if and when a feature is merged into Forge I will remove it from the Forge version of this mod (the Fabric version will keep the feature).
The amount of memory saved will depend on the pack, in version 1.2.0 of the 1.16.4 Direwolf20 pack it reduces the memory usage (after GC) on the title screen roughly from 3.1 GB to 1.1-1.2 GB.
The project picture is a cropped version of this picture.
Can you backport this to [MC version before 1.16]?
No, I won't backport it. I do not consider it worth the effort, and would like to encourage people to use more recent versions.
Should I install this on the server or just on the client?
Some optimizations are client-side only, but there are some rather high-impact optimizations that are relevant on the server too. So you should install it on both sides.