Fidelityfx is platform agnostic so I don't see the issue. It runs on my vega gpu and it runs on rdna 1. So i don't see it suddenly not working with RDNA 2. The question is if they have added anything to it in RDNA 2. MS has said they have added ML hardware to the XSX. That hardware could be used to improve Fidelity FX or AMD may have improved it on their own. We will have to wait for hot chips or for amd to announced RDNA 2.
Here it is on a 5700
Here it is on the vega 56
So like I said , I'm expecting an improved edition of this and with amd's new hardware supporting machine learning i would assume amd would leverage that like Nvidia has.
Now of course this is not as good as DSLL but again it doesn't require any special hardware at this time.