Despite what the official source says, the third stage may happen sooner than expected: as the commenter KY noted, the anti-malware service will stop as early as June 15, 2015.
Whether intentional or not, this was confirmed by my tests as well.
If you already have Microsoft Security Essentials installed, you will continue to receive anti-malware signature updates through July 14, 2015.
There are always reasons why people and organizations can’t upgrade, but to provide further encouragement, Google has announced that Gmail will stop receiving support on Chrome 53 and lower by the end of this year.
By the end of the year, Gmail on Chrome 53 and below will be directed to the HTML5 version.
Starting next week, users with older versions will see a banner at the top of Gmail encouraging them to update.
Funny enough, Microsoft pushes the new SE-version (4.5.216, with nagging screen) through automatic updates, but still has the previous version (4.4.304) available for manual download.
I would not be surprised if they've pushed the 4.5-version only to XP-users.