Maybe this is already know; It was news to me. Reaper 6 works on Windows 98.
I've tried using the Reaper installer, to install the application, without any luck. It had an error writing a png file. Maybe this isn't an issue for anyone else? Can't imagine it is just me. If I use Linux/Wine to install the application (Portable), and copy it over, it works.
You could probably use a standard install, rather then portable, if you made a back up of any added/changed registry values. You'd also need to copy any files added to the Windows Directory. I didn't try this. Portable is fine for me; I don't need the Rewire or ReaAsio stuff (does not install in portable mode).
There are some quirks. I can't resize VST Windows. If you dock the VST you can access it's UI.
I did a quick render, of a simple tune, and it worked.
On my system, it didn't love ASIO4ALL (laptop internal audio). I only had audio inputs; where other ASIO clients have worked fine. But the ASIO driver for my USB worked perfect (input/output).
I do have KernelEx installed. I haven't tried with it disabled. If Reaper 6 works, 5 will also probably work. Certainly, some things are gonna be broken. Reaper 4, and earlier, install fine (from the installers).
Again, sorry if this was already know. I thought it was cool.