If you open a legacy (pre-P11) file in P11 and save it, it saves it in P11 format. Not only the rigging format has changed, but also materials, custom palettes, translations, and information about other new P11 features. To a large degree it has been done in a way where earlier versions will ignore the new features if they are not used.

The focus has always been compatibility to be able to use legacy content in the current version, not on creating content which can be used in older versions.

Yes, it would have been nice to be able to save in P10 format as well, but that same argument would have to be applied to P9 and earlier versions as well. For whatever reason (lack of development time, design restrictions or other reason) that has not been done. Maybe the new development team sees differently.