It's not always obvious what web sites use Shopify. For those sites that Shopify shuts down, the effect will depend on how Shopify goes about it.
1. Total shut down....
If Shopify "kicks sellers off the platform", then the sellers will not be reachable on the web until they move their web site to a new host.
2. Stop processing transactions, but don't disable the web site...
If Shopify stops processing purchases but leaves the web pages live... Customers will be able to view the sellers web pages, but not be able to add items to the cart or process purchases, until the seller moves to another platform.
If you are doing business via Shopify, your web pages are hosted on servers managed by Shopify (or their designee).
Here's the real pain..... Anybody can put up a basic web site in a few minutes. Shopify enables sellers to put up a complex web site that can do complex things like enable the selection of products based on multiple variables (a simple t-shirt purchase requires a buyer to select style, color and size, for example), match that selection to inventory and process payments (which are regulated and secured). Replicating that functionality independently can cost hundreds of thousands of dollars. Moving that functionality to a new platform is not a trivial or inexpensive task, and, the seller runs the risk of being booted from the next platform as well..
Quite a sticky situation.