Had this issue too after using OBS for about 12 years, this one threw me for a loop. I firstly uninstalled the old websocket plugin (I'm on Windows 10), then uninstalled OBS (uninstalled both via the Windows Add/Remove Programs utility). After that I checked my ":\Program Files\" directory for any remaining OBS type files. Then reinstalled the newest version of OBS (V28.1.2 in my case). Rebooted just-in-case, opened OBS and found under the Tools menu the websocket tools. All was right in the world again.
Then went back in and reinstalled other plugins I had: NDI Tools, etc.
Hope that helps!