For the overwhelming majority of website users, there is nothing to worry about and September 30 will be business as usual. Older devices, however, could run into some trouble, much like they did when the AddTrust External CA Root expired back in May. Stripe, Red Hat and Roku all suffered outages as a result.

Source: Lets Encrypts root certificate has expired, and it might break your devices

Related: OpenSSL Client Compatibility Changes for Let’s Encrypt Certificates - API Announcements - Let’s Encrypt Community Support

I’ve run into a few broken things today. If something worked yesterday and it doesn’t today, this might be why.