Odoo is making drastic changes based on feedback from a few users about the current setup — for example, the date format — while ignoring millions of users who are perfectly fine with how things are.
Changing the date format from numbers to letters should have been optional or at least adjustable through a setting.
I noticed this change when upgrading from 18 > 19. We are using SaaS and while I can go into Studio and change every field to the numeric format, it is a giant pain. The Language > Date Format setting doesn't affect the fields.
I agree with OP, changes like this shouldn't be global without a way of reverting globally.