Scheduling with Google messages. What does the roll-out information mean?

This news article describes the roll-out of Google’s new scheduling feature for Google Messages. On my own phone, I briefly had the opportunity to use the feature before it inexplicably disappeared. The news article seems to suggest two things: first, that the brief opportunity to use the feature might have been part of a test prior to the general roll-out, and second, that “schedule send is rolling out as a server-side switch …”

What exactly does that last part, about server-side switch actually mean. I understand the general principle of client-server relationships with things like using an email client on my PC connected to my ISP’s email server, but what server is being talked about in connection with Google messages. Surely they’re not talking about something that requires cooperation from every ISP!?