system
Given the following post, respond as if you're commenting on Reddit:
human
[W40K] What were Human-Eldar interactions like during the Dark Age of Technology and up until Slaanesh?
gpt
The events of the Fall began in earnest about M15, at the same time the that Humanity was beginning to expand. Before this, however, the Eldar were becoming increasingly insular as they turned more towards indulgence. Typically, this causes a shrinking of borders as Imperial citizens move back towards the core.

The most likely sequence of events has the Eldar slowly migrating back towards the core of their Empire (now the Eye of Terror) as early as 10,000 BCE (considering the leisurely pace the Eldar prior to the birth of Slaanesh) though this would have been well progressed by the time of M15, with the majority of the Eldar population concentrated in the Eye. 

Their increasingly insular nature and self indulgent behaviour would have left the Eldar littler consideration of anything beyond their core, but we do know that there was some sporadic contact. The Eldar are described as one of the earliest races Humanity came into contact with, though it is unclear exactly how amiable that contact was. We are also told that, during the Age of Technology, humanities various empires made frequent treaties with alien races. Considering the proximity of the Eldar core, i think it's reasonable to say there was some kind treaty established. 

Anyway... we know the Eldar, at their height, dominated the galaxy, with colonies from one end to the other. At the time of the Age of Technology, they seem to be located almost entirely in the core of their Empire, supporting a the mass migration theory. Likewise, the increasing density of their population explains why Slaanesh caused the Eye of Terror when no other Chaos God did the same. 

TL:DR

The Eldar had abandoned most of the Galaxy, and probably ignored Humanity for the most part by M15. What little contact they may have had would have been a nuisance at worst, a curiosity at best.