I received a reply via e-mail. For future reference:
The unstable API is the only place where the NFT Sales Eligibility API works for the moment, and no definite plans for moving to a stable release. When it does, it will only be with that stable release and forward, it won’t be applied to older dated API versions.> > With that said, most apps can simply use the unstable API for that one call, while using whichever version for the rest of your app. Some development environments may not allow this though, in which case it would require you to update the entire app to use the unstable release.> > The deadline to be checking all shops that are doing any kind of NFT distribution was back on Feb 9 I believe.> > As for the new admin URL, you can just add /nft-application to the end of it (ie https://admin.shopify.com/store/{shop}/nft-application) and you should see the form, assuming the shop is in a supported region, and Shopify Payments is enabled.
Keep in mind that in Node.js it is not possible to change the API version to the unstable API for one call.