Cmd + click places multiple carets on macOS. Apparently – I never touch the mouse when I’m editing in Helix. I don’t know which modifier it is on other platforms.

I’m not sure if replicating Sublime’s Ctrl + D is possible or not, but there are other ways to achieve every use case for it I can immediately think of. e.g. I think I’d typically be doing `<space>h` to select every instance of the symbol under the cursor, or using `s`elect to reduce a selection to a match, possibly yanked and pasted.