While Subaru does not manufacture the glass, they are fully responsible for any defects. It is their job to ensure that the supplier provides adequate glass. Which they have completely failed to do. They are fully aware of the defect in the 2017-18, yet they did absolutely nothing to address the issue on a full vehicle redesign, let alone fix the problem on the existing vehicles.
I work in the automotive industry for a vehicle manufacturer. Our Warranty Department is located on the other side of my cubicle wall. I author the owner's manuals, as well as service bulletins and federal recalls. I work closely with our engineers to correct field issues as we learn about them. To address some of what you said:
Subaru is responsible for making the customer whole in the event of a defect, but that does NOT mean they are responsible for "ensuring that the supplier provides adequate glass..." There aren't Subaru inspectors at the glass plant watching each windshield come off the line. There are spot checks, but every part does not get inspected by Subaru (but the supplier might). Completely failed to do? They're replacing windshields that failed, that's all they're required to do by law.
As far as the carryover of the defect from '17-18 to '19, you do understand that these are two different vehicles. They do not share windshield glass part numbers. They may even use different glass suppliers for each generation. Besides that, once the 2014 forester was released, the work began on the 2019, so they may have been working on the 2019 glass before the '17s were even released.
There is a Subaru logo on the glass. They are responsible for their product. In addition, if the 19's are a problem like the 17's/18's, then that is an even bigger issue, as they were aware of the problem prior.
While they're responsible for their product, just because their logo is on the windshield doesn't mean they manufactured it. We require suppliers mold/cast/print our logos and part numbers on our own proprietary parts, we basically don't manufacture anything, we assemble parts from different suppliers.
Again, '17-18 and 2019 are two different animals. As stated above, they were probably well into the design of the 2019 before the new glass was OK'd for production on the '17s. By then the supply lines were running and it would be cost prohibitive to scrap tens of thousands of windshields for a redesign, when only a fraction of the total population are seeing failures.
Is this a customer dissatisfier? Absolutely! Am I defending Subaru or SoA? NO! But it's really easy to make angry statements and say "They should have known! They should have done better!" when you don't really understand how the automotive manufacturing process works. I do have some empathy for them, but if it was my car I would be livid about the backorder.