At Ignite we announced a major improvement to the way secure external sharing of files and folders works in both OneDrive and SharePoint in Office 365 and we wanted to share what this means for users...
Updated Jun 25, 2020
Version 4.0
Blog Post
Hi Brett Cox. The checkbox you’re seeing does not control whether this feature is enabled or not. Instead, it is an optional feature (off by default) which lets you specificity how often recipients need to re-verify their email. For example if you share with amy@contoso.com and this checkbox is set to 30 days, then when Amy enters in the code to access the file/folder, she will have to do the same thing again 30 days later (even if she selected to be kept signed in). This way, if she loses access to her email account (for example if her employment status changes) then she also loses access to that shared item (because she can’t enter the new code).
Does that make sense?
For site sharing, you can still share externally but for now it will use the previous system that requires that recipients create accounts.