The only known ways to detect the defect involve x-raying the battery or destructive disassembly. They could theoretically send the batteries back to a centralized location for testing and refurbishment with 2019-2021 models, but it isn't clear that would save them any money, and they already promised new batteries to owners of 2017-2019* models.

My understanding is that what they are really working on is a cost effective way to check for the defect on new batteries leaving the LG plant, so they can be confident that the replacements don't have the same problem.

* Some 2019 models were included in first recall, others in the second, based on where the batteries were manufactured.