Is there an open source IO-Link stack available?
The standard is open and explained in detail in IO-Link official website. But it is a little bit complicated, so it requires some effort to write a stack for it.

In order to make an "official" IO-Link product, you have to get your device tested in an approved IO-Link test center, and pay some money for license. In my opinion, because the whole thing is very commercial and benefits very few people, currently the community isn't interested in writing an open source IO-Link stack.