Nobody knows. It is very likely everything pre-RX 400 series will be phased out first, then a year or so later everything pre Navi, but there is precedent for GPUs to be dropped from support while their warranties are still intact (namely the HD 6000 series), and the RX 500 series is near 5 years old.
However, new games like Cyberpunk 2077 really show how...inadequate anything pre-Navi is these days, with 1920x1080 medium details only amounting to 48fps average on the RX 590, so their usefulness going forward is only going to continue to be more and more limited, and this, not official driver support, is what will kill anything pre-RDNA2 in a very short time.
https://www.tomshardware.com/news/cyberpunk-2077-pc-benchmarks-settings-performance-analysis
