Microsoft has officially stopped selling licenses for Windows 7 and Windows 8.1 to PC manufacturers. With this move, partners like Dell and HP will no longer be able to sell new PCs running either previous version on Windows, having instead moved fully to selling PCs pre-loaded Windows 10.
The end date for sales to OEM partners has been known for a while, having been listed on Microsoft's Windows lifecycle page (via VentureBeat). While Microsoft had previously ceased selling licenses for Windows 7 Home Basic, Home Premium and Ultimate to manufacturers in 2014, Windows 7 Professional continued to be available alongside Windows 8.1.
It should be noted that mainstream support — which includes patches for vulnerabilities and the like — for Windows 8.1 is still set to continue through January 9 2018, while mainstream support for Windows 7 ended in January of 2015.
In any case, if you're planning to buy a PC in the near future, it's now a near certainty that it will be packing Windows 10.