I could make this for you. Seems relatively simple:
1) Need to add a "follow" button to the UI (for signed in users)
2) Whenever you create a new product, send an email to all "following" users
Shopify already has support for logins + product create webhooks. It would be a relatively easy project. Feel free to shoot me an email: email@example.com
I'm happy to build it as a public app, so no cost for private development.