Currently there is no functionality that enables a community manager to set a community in "Maintenance Mode". This functionality should enable a Maintenance Page to be configured and set up from the community builder and the "Under Maintenance" option should be available from the community administration pages. Once a community is set as "Under Maintenance", all current active users should be logged out and redirected to the maintenance page. All users trying to access the community should be redirected to the maintenance page as well. Once the "Under Maintenance" mode has been disabled, the community funcitonalities should be restored.
Currently, deactivating the community just leaves the user on a blank loading page with no message whatsoever, so users will feel like their community is broken.
This is a vital functionality when planning major realeses that have an impact communities, specifically on communitys that have custom solutions applied to them, such as Custom Lightning Components and Specific CSS, Themes and Styles applied to them. This can provide a time window in which the deployment team can run any manual steps and smoke testing needed to ensure that all the changes are working properly after the deployment.
A similar functionality was previously availabe on Sites and I think should be a must for big communities.