Checkboxes

If you just want a way to toggle the selection of items without deselecting others, and without having to hold Ctrl, you can use the middle mouse button for that in Opus.

(Power Mode also lets you make the left button do the same thing, but that won't affect other modes like Thumbnails.)

There is also a checkbox mode, but it's slightly different to (and predates) the one you might be familiar with from Explorer. It's more about being able to separate the list of files which commands will act on from the file which is open in the viewer pane:

https://www.gpsoft.com.au/help/opus12/index.html#!Documents/Checkbox_Mode.htm