Yet again you are wrong, Windows 1809 is supported until November 2020.
zelda412 Nobody knows how long they will be supported, although the term "support" has a couple of different meanings. If you mean, "How long will the RX 580 continue to receive performance and feature updates", then quite likely not much longer, since if you look at the release notes mostly everything since the RX 5000 series was released is focused on them. Vega users experienced this as well despite having much more expensive cards. AMD is also set to release their first ray tracing capable cards later this year, so their resources are going to be heavily focused on those.
If you mean, "How long will the RX 580 continue to receive new driver packages", then likely for at least two more years. The HD 7000 series through the R300/Fury series will be the next batch to be moved to legacy support.
There is precedent, however, that cards not even out of warranty have been moved to legacy support, namely the HD 6000 series, because they were not DirectX 12 capable. A similar thing may be about to happen as DirectX is about to receive its largest update in years, DirectX 12 Ultimate, which will have features only supported by the next generation of cards coming later this year, RDNA2, but again if a card doesn't receive any updates, is it really supported?
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