"This selected collection might contain products that are not publically available. Products must be available in the sales channel "Online Store" in order to be included in the feed. The selected collection has currently xxx publically available products."
[Update] The new warning says: Not all products are available in the in the "Online Store" sales channel!
It's an alert to make you aware that the collection you have chosen might contain products that are not publicly available and thus won't be synced.
You need to check the "missing" items if they are available online. They need to be active and managed for the Online store channel on your Shopify product page.
Hint: You could try to select all products on your Shopify backend (click on the 'Select all in this store' link), press the ... dots, and then click on 'Include in sales channels'.
