Serv-U MFT already has options to disconnect active user sessions, abort, spy & chat, and broadcast. But several times I've wished I could prevent or block any new sessions to allow a current session to finish and disconnect before I stop and update the server.
This function would be either a toggle that prevents any new sessions while the slider switch is "on," or a timed function that expires after a configurable amount of time or until the server restarts. Either way, this function would not disconnect current sessions.
USE CASE:
I want to apply an urgent Hotfix to Serv-U. I do not have time to send out a communication for a planned outage, and a few users are transferring files. I activate the "Prevent New Sessions" toggle, then use the "Spy & Chat" feature to ask the current users to finish up and disconnect when they are done. "Prevent New Sessions" ensures nobody can sign in while I wait for these users to finish, then once they disconnect I can stop the server and apply updates.
Low-effort screenshot with idea:

EXTRA CREDIT:
SolarWinds, you get a guaranteed A+ on this feature implementation if you also add a feature that notifies admins via email when there are no active sessions. This may require a separate feature request to add SMTP notification settings; if that's the case I'll make that feature request too.