Prevent access to the dashboard(wp-admin) for selected subsites. Only super admin can access the dashboard.
In Network Admin->Sites, toggle the subsite you want to put in maintenance mode.
Chechboxes are added to Network Admin->Sites using the WP_List_Table_Custom_Toggle_Column class.
Subsite Maintenance Mode is copyright 2021 Per Soderlind
Subsite Maintenance Mode is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version.
Subsite Maintenance Mode is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU Lesser General Public License along with the Extension. If not, see http://www.gnu.org/licenses/.