Exactly. I tried to hang Knauf fluffy rockwool on walls and it was very useful. But totally not acceptable for normal life, of course.
My 1st reflection on one wall is door and window blinds, so I need to use something rigid in their frames.
In the room described in my link, there is no side-wall treatment for 1. reflection, due to practical issues. And it does show slightly worse early-reflection attenuation compared to what has been achieved in other rooms. But sometimes you have to compromise, and the overall result is still very acceptable.