Well they never had a 5700XT in one of their machines and brought support in 10.15.2 anyway because :
1 : they don't need to bother, it's AMD doing the work getting the drivers ready.
2 : those drivers support a whole family of GPU's even if only one specific reference (mobile SKU as an example) is built into a mac.
I doubt they'll let down those who poured $5000 into a high-end intel MacMini and cut them from a decent eGPU in the future.
I also doubt they'll let down MacPro 7.1 owners, a lot of them being angry MacPro 6.1 customers because of lack of upgradabilty.
RDNA2 cards will come to the mac one way or another, does not matter if it's not specifically the 6800XT it will get support at the same time.