My biggest question is why are such changes shipped without any notice or a changelog entry?
Apps with decent scale that are making use of line item properties get flooded with support tickets. 
@Liam-Shopify @Alan_G Please forward this to the right team.
In combination with the long-standing request to allow editing of line item properties: [Feature request] Allow customAttributes input (line item properties) for orderEdit API
Also hiding properties starting with “_” by default would make so much sense. The UI could add an eye icon to toggle the visibility.