Basically, anyone who tries to connect will get that warning until the certificate is renewed on the server.

Here is an explanation I found on Quora.

When a user tries to access a website with an expired certificate, the web browser will typically display a warning message that the website is not secure. This is because the certificate is used to establish a secure connection between the website and the user's browser, and without a valid certificate, the user's browser cannot ensure that the website is genuine and secure.

If a user chooses to proceed to the website despite the warning, their browser may allow them to do so, but the website will be considered insecure. This means that any information transmitted between the user's browser and the website may be intercepted and viewed by a third party.

Hackers can take advantage of this situation by setting up fake websites that look similar to the original ones and try to trick users into entering their sensitive information. In this case, the expired certificate provides a false sense of security and makes it easier for hackers to exploit the users.