We used to have the above Hoover washer-dryer when I was a kid. The instructions said that no lint filter was required if the machine was vented outside. Ours was indeed permanently vented out through the wall, so we didn’t use the filter. The lint was blown out through the vent.

However, if you were using it without venting it outside, the instructions advised that the filter be inserted into the air outlet on the back of the machine (at the very top left, when viewed from behind the machine). This somewhat awkward arrangement would require access to the rear of the machine after every cycle to clean the filter. You could reach over the top to remove the filter without too much trouble, as long as you had installed the air outlet cover to direct the air upwards out of the back of the machine. But you wouldn’t be able to cover the machine or build it in if you were using the filter.