At the moment (January 2021, Jahia releases 8.0.1.0 and 7.3.6.0), the property maintenanceMode is not being handled by Jahia (cf https://jira.jahia.org/browse/QA-13569).
Consequently, how can we activate the maintenance mode?
There are 3 ways:
curl -s -u 'USER:PASSWORD' -XGET "JAHIA_URL/modules/tools/maintenance.jsp?maintenance=true"
ssh -p 8101 jahia@localhost -C "dx:maintenance on"