The ArcGIS Pro design team uses a couple of tools for wireframing. Our go to tool is Figma. We use Figma for brainstorming new features, creating initial concepts and finally also making hi-fidelity wireframes. Another tool we frequently use is Balsamiq. This is simple and easy tool for wireframing but it is definitely not a tool for hi-fidelity designs. Another option that we do not use internally but is a good wireframing tool is "Lunacy". This is a free wireframing tool comparable to Sketch and Figma. This is a good tool to get started with.
