It's deprecated in PHP 7.2+ to use strings.

It now requires Feeds >=7.x-2.0-alpha8.

If an invalid product type is used in the source, a product of the default
type will be created.

See https://www.drupal.org/node/2426449 for more details.
Commerce Feeds needs a similar change, see #2635548.

There's also an overhaul of the code: rather than reimplementing
FeedsProcessor::process() override newEntity().