A year ago when Microsoft released the Windows 10 May 2019 Update, it announced a fundamental change in how it delivers feature updates. It would no longer force users to upgrade to a new feature update, instead it promised to use a system where you'd have to specifically opt into installing it.
There was one caveat though. If you were still using a version of the OS that was nearing the end of support, you'd be automatically upgraded to the newest version of Windows 10. That's happening now for users of Windows 10 version 1809, the company confirmed on its Windows 10 version 2004 dashboard. Back in May, the company had updated the version 1809 dashboard to reflect that this would happen in June, and now the time is here.
Windows 10 version 1809, or the Windows 10 October 2018 Update, was a troubled one. Microsoft skipped right over the Release Preview ring of the Windows Insider Program and just released it. It turned out to be so bad that it had to pull the update four days later, not re-releasing it for over a month. Eventually, things evened out and according to AdDuplex, the October 2018 Update is still currently installed on 8.2% of Windows 10 PCs.
If you're still using Windows 10 version 1809, there's nothing that you need to do right now, as long as you're OK with being upgraded to version 2004. However, if you want something that's been out in the public for a bit longer and therefore a little more stable, you can upgrade your PC to version 1903 or 1909 instead.