The SRL folder, is indeed, a decoy. I'm not sure why it's still there by default - it housed SRL-5, many moons ago when that was what we used.
You are correct in thinking that Includes/SRL (SRL-5) has been replaced by Includes/srl-6 (SRL-6).
All scripts including srl/srl.simba will
not work with SRL-6, they are completely different includes and even use seperate interpreters (PascalScript and Lape, respectively).
***
Now for the solution: to accomplish what you need; developing and running scripts for private servers (under SRL-5), you will need to download the old include from
here and place it in Includes/SRL.
After that, srl/srl.simba will actually exist, be included into scripts, and function.
Note that no part of SRL-5 will work with RuneScape 3
or Old School RuneScape. SRL-5 was left behind after the move from RS2's EoC -> RS3 (new interface system, etc).
However, most RS private servers are of a revision that (somewhat) resembles RS2, and that is why most private server scripts are written primarily under SRL-5.