If you have a cPOS (or mPOS) with a "mobile" type layout (the "compact" one) applied, and you go to the Inventory Lookup screen you will see that the option to "View All Variants" is not present.

With a regular POS layout, you are presented with a choice of "View All Variants" and "Select a Variant", but the view all one is missing in the compact layout view.
Under Review
Ideas Administrator

Thank you for this product suggestion! We will continue to monitor the idea and votes to determine its consideration / prioritization in our future release. - Boyce Zhu, Senior Program Manager, Dynamics 365 Commerce