Starting April 1, 2019, it will no longer be possible to restrict an entire organization (tenant) to classic mode for lists and libraries. Lists and libraries may still use classic mode using the granular opt-out switches that we provide at the site collection, site, list, and library levels. Additionally, lists that use certain features and customizations that are not supported by modern will still be automatically switched to classic mode.

After April 1, lists and libraries that are in classic mode as a result of tenant opt-out will automatically be shifted to modern. Users will benefit from enhancements and new features such as attention views, PowerApps integration, Flow, column formatting, and the filters pane.

What should I do to prepare for this change?
The SharePoint Modernization scanner identifies sites and lists that have customizations that are not supported by modern UI. Although many of these lists will automatically remain in classic mode even after this change, you may wish to keep some sites running entirely in classic to avoid users switching between different experience modes within a single site.

Administrators can use a PowerShell script to enable or disable the modern experience for a single site collection or for a list of site collections as provided by the SharePoint Modernization scanner. Alternately, users can still use the “return to classic” option on modern views of lists or libraries to temporary return to classic, and list owners can use List Settings to configure that list to use the classic experience for all users.

PowerShell scripts to opt out of the modern list and library experience: https://docs.microsoft.com/en-us/sharepoint/dev/transform/modernize-userinterface-lists-and-libraries-optout

Source: Office 365 Message Center