Proxy is on the near term road map, but it must be accompanied by conversion to edit-friendly (optimized media in other programs) as a strongly encouraged option. Otherwise. there will be many support issues where the final output does not look like the version previewed (edits in the wrong place, files not seeking good, audio artifacts, etc.). People throw all sorts of garbage media at Shotcut. At least currently people can usually experience some issues in preview and convert manually to resolve. Proxy media will mask all the possible issues that will occur at the time of export.
One of the design goals of Shotcut is that you can quickly and easily drag-n-drop files from a file manager to the player to preview them before deciding to include them your project. Displaying a modal dialog for all HD+ is a major impediment to that workflow, and I will not be doing that. Proxy media will be an option in the New Project view and convert basically all media at the time it is added to either Playlist or Timelime. Maybe there will also be a per-file option somewhere when not using a project folder, but I have yet to fully design this.