So far SharePoint Online provided the possibility to enable / disable the use of custom scripts at the tenant level … and I say “so far” because apparently Microsoft is removing this option (at least I cannot find it in my tenants SharePoint Online which are configured "First release" mode). Indeed, today when I accessed to the “Settings” page in the SharePoint Online Administration I couldn’t find any option to enable / disable custom scripts at the tenant level. I can guess why Microsoft has decided to remove this option due to the many problems caused to existing SharePoint sites when disabling this feature an causing a lot of side effects as explained in the following support article: https://support.office.com/en-us/article/Turn-scripting-capabilities-on-and-off-1f2c515f-5d7e-448a-9fd7-835da935584f?ui=en-US&rs=en-US&ad=US