Default patterns are in the works (see https://github.com/syncthing/syncthing/pull/7530). Global patterns (i.e. editable patterns applied to all folders) would be cool, but someone needs to do all the work to implement them.
As for per-OS ignore patterns, there is likely going to be some controversy with them. For example, there are way more things to ignore in Windows than just the two you’ve mentioned above (e.g. the Recycle Bin, Windows backup folders, and many other system folders located in the drive root). If ignore patterns were always filled with them by default, it would create a lot of clutter, which many people will likely not enjoy. Of course, such settings could be configurable, but still, a lot of work will be required to implement all of this.