Erlang, 180 bytes, 11 versions, score 16.36

20-length([A||A<-[schedulers,c_compiler_used,cpu_topology,snifs,dynamic_trace,port_count,nif_version,end_time,max_heap_size,atom_count],{'EXIT',_}<-[catch erlang:system_info(A)]]). 

With indentation and line breaks:

20-length([A||A<- [schedulers, c_compiler_used, cpu_topology, snifs, dynamic_trace, port_count, nif_version, end_time, max_heap_size, atom_count], {'EXIT',_}<-[catch erlang:system_info(A)]]). 

Tested on one minor release of each major version since 10:

• R10B-9
• R11B-5
• R12B-5
• R13B04
• R14B04
• R15B03
• R16B03
• 17.5.6.2
• 18.2.1
• 19.2
• 20.0

The idea is that every major release has added at least one new allowable argument for the function erlang:system_info, so let's try the ones in the list, count how many of them fail, and subtract the number of failures from 20, which is the current version.