Neither can I; there's no "わたしあと" in Jisho or d.goo. Just searching the net returns this page: https://www.kankou-gifu.jp/spot/detail_6769.html where it's written as 渡し跡, which would translate to something closer to "the ruins of a crossing" and seems to be used literally for a marker designating the former location of a river crossing ferry.

Maybe this isn't Japanese, or maybe it was made up by a non-Japanese speaker who searched for the words "me" and "after" in an E-to-J dictionary and then mashed them together. I'm leaning towards the latter.