It's assumed they did, though I don't think any of the incarnations of Elite not the official (canonical) novels and stories mention where it is or it's physical description. I seem to recall one of the short stories suggesting the Thargoids may even be a nomadic species, moving through the galaxy in their fleet and never settling in one place for very long - if so, then perhaps some other even more xenophobic alien race destroyed their original homeworld long ago.
I don't know if there ever was a formal "treaty". All we know about the post-FFE-pre-FD timeline, as far as the "official canon" is concerned, are the in-game newspaper articles (the FFE precursor to Galnet) that appeared long after the main FFE storyline was completed. You can read them on this old webpage:
http://www.jades.org/3255.htm
The FFE game was notoriously glitchy, and one of the glitches was how the newspapers worked, or rather didn't work properly, with articles sometimes appearing in random (non-chronological) order and game-relevant place-names replaced with the name of the planet you happened to be on when reading the article. It's uncertain whether or not anyone was ever actually supposed to read those articles, or if they were part of a planned continuation of "the plot" that was abandoned. So it's uncertain exactly how "canonical" they should be considered.