I searched (using Google, the search engine on the board isn't very good, IMO) and didn't find much.
There is this post (no relish mentioned)
https://www.quiltingboard.com/t-129435-1.htm
Another post had a link to a site with lots of rhubarb recipes, scroll down for the relish.
https://www.rhubarbinfo.com/2020/03/...ipes.html#more