The 64 bit version of the plugin is available for GIMP and XNViewMP.
Note that you need the 64bit version of the host applications too.
XNViewMP comes as 64bit and 32bit, so technically the 32bit version should work with the 32bit version of the plugin.
The installer should detect GIMP 2.10 and XNViewMP. If you used portable versions, you need to manually choose the Plugins/Addons folder.
The installer is now HiDPI aware, and the text will look crisp on such displays.
Note: IrfanView has discontinued support for RIOT. The IrfanView author insists on bundling RIOT and insists on certain size/structure of the plugin. If you want RIOT back you can write him to request to add back the support and not care about the size or structure of RIOT.