For now, the option has started turning up on preview versions of Windows 10 1903 and the May 2019 Update.
A screenshot from a Reddit user shows that below the 'Check for updates' button, and within the section titled 'Additional updates available', there's the new option to 'Download and install now'. In this case, the 'Download and install now option' has been made available for a cumulative update.
Choosing to download and install when offered a feature update is taken as confirmation that the user wants that update. From that point on, the feature update can then only be paused for up to 35 days.
By not clicking on 'Download and install now', the new feature update can be avoided so long as the version of Windows 10 currently running is supported and not nearing end of support.
Microsoft hasn't specified how close it means by "nearing". However, if it detects machines running Windows in that state, they will automatically be pushed the next feature update.
The extra control here means that users can click 'Check for updates' to get updates besides the feature update, while the 'Download and install now' option gives users control over the feature update.
Microsoft will also extend this 'Download and install now' control to Windows 10 versions 1803 and 1809 by late May, which is when it plans to release Windows 10 1903 to the general public.