productID (Domain: Product, Range: Text):

The product identifier, such as ISBN. For example: meta itemprop="productID" content="isbn:123-456-789".

Description

I think it could use a more detailed description about the expected value. No expectation/recommendation, or does it have to use some format (like prefix:value)?

And maybe it would be better to keep the description syntax-neutral (i.e., removing the Microdata example)?

See also @danbri’s comment from 2014:

http://schema.org/productID is vague too; what should it contain?

"What are the allowed prefixes? isbn? gtin? ean? jan? upc? sku? mpn? is the colon mandatory? Is the prefix case sensitive?"
(Matthias)

Some discussion (from 2015) about this starting at #428 (comment)

Sub-properties

Currently no property has productID as super-property. Would it work to add the "gtin" properties as sub-properties? They are also defined for Offer/Demand, but productID isn’t.

See also @mfhepp’s comment from 2015:

[…] (same as productID is a superproperty to gtin13 etc.)

Note that the superproperty relations for productID are still missing in schema.org (but will follow)