Walls with openings, such as engine room louvers or building windows, require a balance between permeability and sound insulation. Generally, increasing the aperture area to reduce flow resistance leads to a degradation in sound transmission loss. While previous studies have utilized topology optimization to design acoustic metasurfaces to address this trade-off, they often relied on predefined opening areas. This constraint limits design flexibility, as simply minimizing the acoustic power in the outlet domain typically results in a trivial solution where the design domain is completely closed. In this study, we develop a topology optimization method for designing acoustic metasurfaces with openings without predefining the aperture area. Our approach maximizes the imaginary part of the wavenumber within the design domain, which inherently leads to high transmission loss while preventing the domain from closing. Furthermore, we clarify the underlying mechanism by which soft boundaries, formed through this method, reduce radiated acoustic power. We designed acoustic metasurfaces for both single and multiple frequencies using a level-set based topology optimization method. Sound sources were defined as incident waves from multiple angles using a Gaussian beam function. To validate the robustness of the method, the initial configurations were set as completely closed domains. The resulting performances were evaluated by comparing the radiated power against simple apertures with equivalent opening areas.