Yeah I've looked that up before, and from what I recall reading, the antibody tests only work for "recent" infections in the last few months, not something that may have happened (say) a year or more ago. I don't remember reading that any test works for non-recent infections.
Yeah, there's a limited window over which antibody testing is useful. I hadn't heard that it was only a few months, but that wouldn't be shocking, especially if you want a low rate of false negatives. Hopefully someone reading this knows more than me about approaches to testing that might work over longer time scales and not be confused by vaccination.
Actually I just Googled again, and they simply say the precise duration is unknown [1]:
> IgG antibodies, including IgG against the S and N proteins, persist for at least several months in most persons, but the precise duration of time that antibodies persist after infection is unknown.
