Very old software may gain an edge. However, that takes a while and depends on the number of people using the old software as well. For instance, Windows XP was

Very old software may gain an edge. However, that takes a while and depends on the number of people using the old software as well. For instance, Windows XP was a big target for decades after it had been superseded by newer versions of windows. If your software stack is from the 90s, I'd say you might just getting obscure enough by this point. Scanners are frequently not looking for the breakthrough 0-day vulnerabilities, but just for the low hanging fruit of old and well known vulnerabilities.