Similar to full mistborns: in arc 1, they existed rare, but still present. In arc 2 it was stated that Allomancy had gotten weaker over time and that there had not been any full mistborn in quite a long while, only mistings, which are implied to be the lesser variant of Allomancy/weaker genetics mistborn essentially. Feruchemy is implied to have inheritance patterns directly mirroring that of Allomancy, so it can be inferred that the lack of Ferrings in arc 1 can be explained by "Feruchemy was stronger back then".
We didn't see much of feruchemy period in mistborn books 1-3 and as such we have no confirmation that ferrings existed (or not, I guess) at that time.