Yeah, but Syncthing offers much more than simple synchronisation. Is there any other software that can copy files between multiple devices while also being able to automatically establish a connection and reconnect even if the devices are used in completely different networks (or even on the move, i.e. using mobile networks)? I personally haven’t seen anything like it, so it’s no surprise people want to use Syncthing for various different purposes (or in other words, adapt the software to their needs).
I wouldn’t have suggested the setting if it had been the typical one-way synchronisation case, where files deleted on one side should remain on the other, but here, where the files need to stay on both sides, I can’t think of anything else (other than not using Syncthing at all). Perhaps one could write some scripts to move the files to different folders or add ignore patterns automatically, but we’re talking about a phone, where running scheduled scripts and such isn’t exactly trivial.
Alternatively, you could use Versioning to keep the deleted files on both devices, but then everything will be split into two different folders, i.e. the original one and