According to the documentation, Polybrush, the tool for mesh painting, sculpting, and geo-scattering, is deprecated and no longer supported in Unity 6.3 and later. In response, AdamNapper launched a replacement called RealBlend, allowing you to create or modify meshes without needing to switch back and forth with Blender, paint meshes directly within Unity, and sculpt geometry in-editor.
RealBlend offers 5-layer blending for HDRP and optimized 3-layer blending for URP and Built-In pipelines. It’s currently in beta and under active development, so all feedback is highly appreciated. You can learn more and contact the developer on Reddit.