It's a popular ink, IIRC from Goulet Pen's list-by-popularity.

goes to look

https://www.gouletpens.com/collections/all-ink

Yeah, on the first page, #8 best-selling, and #1 is "send me a random ink sample", so I'm not sure that that should count; probably should be #7.

I think I have some sitting around myself, but haven't gotten around to trying it.

https://mountainofink.com/blog/diamine-ancient-copper

EDIT: Actually, that's probably not the best ranking to use, since that includes sample vials, and most people getting sample vials are probably just trying it, not necessarily set on using a bunch of it.

But on the list of bottled ink sales, it's #18, which is still pretty high:

https://www.gouletpens.com/collections/all-ink?sort_by=best-selling&filter.p.product_type=Bottled+Ink&filter.v.price.gte=&filter.v.price.lte=