Stash tab purchases made in Path of Exile 1 will be accessible in Path of Exile 2, where applicable. For instance, a special league-related stash tab might not be available in Path of Exile 2 if that content doesn't exist there.
GGG will update relevant stash tabs in Path of Exile 2, and tabs like the currency stash tab will hold different items compared to Path of Exile 1.
When you log into a separate platform where you do not own any stash tabs, they become remove-only.