These days modern filesystems are less prone to fragment files and SSDs are much faster at pulling together the content of file even if it is fragmented, but the impression that a lot of us still have is that windows will slow down over time.
So can Windows slow down over time these days? I'd say that it's less prone to just slowing down due to standard use than it used to be, but that we now have a far more complex environment which is constantly updating due to features like Windows Update. When combined with third-party applications also automatically updating themselves then there's a good chance that perceived performance of the machine will change overtime (I say perceived as sometimes it's just the one application that has slowed down and the rest of the processes running on the machine are performing fine).
Windows can work for years without slowing down, but sadly that requires expertise to maintain. All my relatives who ask to “fix” their laptop don’t have it. And all I do to make them happy is clearing hk{cu,lm}/…/run{,once}, uninstalling “additional useful” software, and that’s mostly it. No background activity - no slowdown.
Sometimes very weak windows computers fall into an update loop when they can’t install updates in one session and are turned off before they have a chance to break out of it. In this case I recommend leaving it on for a day (night), for it to do everything it wants to.