This extension quickly saves the model in a legacy SketchUp version. In contrast to utilizing the native Save As... command you don't need to accept the file location and confirm overwrite on each save.
If you have colleges stuck in older versions or want your files to be accessible using SketchUp Make, which stopped being developed after version 2017, you may find this extension useful.
If the file is already saved, this command will function similarly to the native Save command, and save the file to the same location as the same SketchUp. If the file has already been saved to a newer version than you want to save as, first use native Save As and select what version you want to use.
Activated from File > Save in Legacy Format.
Source code available at Github.