"In Front" in Cycles I'd like to render a certain object so that it is visible no matter what is between it and the camera, rather like the "In Front" viewport option (formerly "X-ray").

Previously this would be done with Render Layers, but since v2.80 has sort of given those the axe, that is hardly an option.

How would I accomplish this effect in a Cycles render in 2.80 without render layers?
You can still do the same thing in the compositor with View Layers. You need to create collections for the objects you want to render in front / behind, as well as new view layers (all buttons for this are located at the upper right by default). You can then go through each view layer, right click the collections you DONT want to see with it, "View layer > Disable from View Layer". View layers should work the same way as render layers prior.