If you want the workflow to be non-destructive, head into the Object data properties tab, click on the Normals tab and click Auto Smooth. That should give you a sharp edge along the cut.
If you want to add a bevel back into your shape you can add a bevel modifier with the angle limit method and then add a weighted normal modifier which should tidy up the weird artefacts.
Note that the bevel modifier modifier may mess up other parts of your geometry and I don't know of a way to apply it to only the boolean cut. For your situation it works just fine. You should also note that the shading here is not as good as if you applied the boolean and modified the mesh as recommended by moonboots. Using a matcap you can clearly see the artefacts this leaves. You may be able to adjust the bevel and weighted normal settings to fix these artefacts, though depending on what you want to use the model for they may not be a problem.
You can also use the non destructive method to get you 95% of the way there, then apply the boolean and tidy up the mesh when you have finished the model.
Keep going! Blender can do some amazing stuff :)
In your case using a boolean to dig this slot seems like the thing to do. Maybe first simplify your topology, i.e. delete one edge loop out of 2 like this (ring select edge loops, then checker deselect, then select edge loops then dissolve edge loops):
Once you apply the Boolean modifier you have this shape:
You need to work on it to have a clean topology and a sharp angle, begin by creating a bevel along the slope:
You must end up with something like this, a topology with quads only:
Here is what it gives with a Subdivision Surface modifier: