VSR is AMD's answer to NVIDIA's Dynamic Super Resolution (DSR) technology, both technologies are intended to render games at a higher resolution than the native resolution of your display. The image is then downscaled, resulting in higher visual quality with smoother textures and edges. AMD says the result is similar to Super Sampling Anti-Aliasing (SSAA), without requiring the game to support this mode.
AMD had to step in with the support for more models, as NVIDIA’s DSR is already widely supported feature. Although AMD quickly copied the idea of virtual oversampling, it would only be supported by Radeon R9 290 and R9 285 series at the time. Those two graphics processors will also be used in Radeon 300 series, so seeing Radeon R9 390 and R9 380 on support list is not surprising.
What is also not surprising, but definitely interesting, is that AMD managed to include Bonaire and Pitcairn-based Radeon R7 360 and R7 370 series to this list. VSR is without a doubt much easier to implement than all in-house features like FreeSync or TrueAudio, so I wouldn’t see this as an indicator for other technology to be ported into new series, but hopefully I’m wrong.