While using the Site Closure policy to set the site collections to Read-Only the banner message is not correct.

While the Modern pages display the message: "SharePoint sites are read-only right now while we do some maintenance. We apologize for the inconvenience." the Classic pages show "This site is read only at the site collection administrator's request."

The later is more reflective of what we're trying to achieve.

The procedure I'm referring to is: https://support.office.com/en-us/article/use-policies-for-site-closure-and-deletion-a8280d82-27fd-48c5-9adf-8a5431208ba5