As far as I know AMD is still supporting all RX Series GPU cards including the 400,500, & 600 series GPU cards.
The problem is the latest AMD drivers are optimized and compatible only for AMD GPU Cards 5000 series or newer GPU cards.
So you just need to wait until AMD comes out with newer AMD drivers for the RX 400,500, 600, & Vega GPU cards.
Seems like AMD is putting higher priority for the RX 5000 series and newer GPU cards at the moment so possibly AMD might be slower in producing newer AMD drivers for AMD Cards older than the RX 5000 series.
NOTE: Several other Users have opened threads here at AMD asking the exact same question. If AMD did put the RX 400,500,600 & Vega GPU cards to legacy they would have advertised that fact when you go to the release notes of the driver or to the AMD download page for your GPU card.
If you go to any Non-RX AMD driver you will see that AMD has a note saying that the GPU card is in legacy status and not supported with AMD driver updates anymore.