I have a mod, originally compiled in 1.9.4, which is still usable in 10.2 (which is great, one less version to maintain). However, when I try to run in 1.11, I get a "does not wish to run in Minecraft version Minecraft 1.11" error, without a reason. There are no explicit version requirements in the mod code. How is Forge deciding which mod jars are runnable in 1.11, and is there any way to share a jar across that range?

Short answer no.

 

There were too many breaking changes in 1.11 for it be be feasible for the jar compiled in 1.9.4 to work on 1.11.

 

The 1.9.4 to 1.10.2 changes were small and didn't touch anything fundamentally so it was possible and forge special cased that mods would work between those versions.

 

Forge decides which version a mod works on by saying that the version it is compiled against unless stated otherwise by the mod(or forge) that that mc version is the one that it works on.

McJty released something called "compatLayer" that allows to support 1.11 and 1.10 with the ItemStack changes.

 

 

This should make it possible to just have to compile it once for 1.11 and once for 1.10/1.9.

I seem to have solved my own problem. I added acceptedMinecraftVersions = "[1.9.4,1.11]" to my @Mod annotation, and the mod attempts to run on all three versions. (The mods are a range, not a list, which tripped me up for a while).

 

Since this mod (Geographicraft) interacts only with the GenLayer stack, which is very stable over Minecraft versions (it hasn't changed since 1.8 ) it actually can run on all three versions.