If you’re seeing features appear in SharePoint Online that haven’t been announced, they may be arriving in your sites due to the following setting:
This setting is found in the SharePoint Admin Center > Settings – https://tenant-admin.sharepoint.com/_layouts/15/online/TenantSettings.aspx
The SharePoint Online Preview Features setting allow features to appear in your sites that are almost ready for release. This post from community.office365.com explains further, though not a clear explanation.
First Release on the other hand:
“… gives customers who opt-in the opportunity to get a small selection of significant enhancements to the Office 365 user experience, SharePoint Online, and Exchange Online first.”
Standard release group (First Release setting turned off) “…will receive the new feature updates a minimum of two weeks after availability in First Release” – Moving to the front of the line with First Release, “Improving visibility to service updates.” Office blog post.
It would pay to check if you have enabled the Preview Features for SharePoint Online. I’m going to leave mine turned on so I can experience changes early.