Language change is neither progress or decay; it is simply change and is happening all the time. New words are constantly being coined and come into regular use, while some gradually disappear from favour with equal frequency.
One such example of the latter is ‘seldom’. It’s a handy word, being shorthand for ‘not very often’ and keeping sentences simpler because you don’t need to build a negative in. For example, you could say ‘you seldom hear that’, which is a good bit easier and more economical that ‘you don’t hear that very often.’
One occasionally hears middle-aged or older folk using the word, but never young people. Despite belonging to the former group, I don’t think I’ve ever used the word. It seems old-fashioned and a touch quaint to me, which befits its somewhat ancient and quirky origins.
I put it alongside that other old-fashioned word which also saves you a negative, namely ‘lest’. I give it as another example – lest you forget :-).