Motivation: you can have 20 upgradeSteps, execute just the last one and it's impossible to know which upgradeSteps before the last one were run or not.
Since you already have an upgradeStep internal id, the idea is to have a persisted list of timestamps when that id was executed/imported.
We started the discussion here https://github.com/zopefoundation/Products.GenericSetup/issues/36 but decided to post here as well since more people from Plone community can give an opinion or help with the code.