Not Invented Now syndrome is the tendency to dismiss technologies simply because they are not the newest hip thing. Sometimes this argument is phrased as "X is dead" or some equivalent. This ignores any lessons which may be learned from those older technologies and there are always lessons.

The old systems have always tackled intrinsic domain problems, such as how to keep threads of discussion straight within a large public forum with hundreds of participants, which just keep on coming up. The old systems have also tried most, not all but most, of the possible solutions to this problem. Ignoring the wealth of research done into the intrinsic problems, especially the ones which aren't immediately obvious, just results in new systems wasting time dealing with well known and understood problems. These are the positive lessons.

There are also negative lessons to be learned from old technologies. It can take decades to determine what just isn't working well. Often you can see this in the reasoning behind why a technology was left behind. These are the only problems within a domain that a new development effort should be working to fix. Sometimes these failures of the old system are due to technical limitations at the time and sometimes due to unforeseen use cases or issues which strained the original architecture. Failure to note these failings just causes new systems to leave consideration and solution of these issues until the architecture has been set and only inferior bolt-on fixes can be applied. SPAM is one good example of this. There are methods of drastically reducing SPAM which require core changes to the design of communication systems. A new system which fails to consider SPAM in its design from the outset is stuck with inferior solutions.

If you don't know history you are doomed to repeat it may be a cliche, but cliches only exist because there is some truth to them.