I do not think Slot availability will be a big issue at Airports for a while. Airlines will not be doing for example 10 return flights a day to X destination when reality is they are at 50% capacity.
We will be in a new normality at the end of this, so going back to where we were in February will not be happening.
Airlines will not be trying to hold onto and pay for slots if they cannot use them for a year, especially when lots of current airlines will not exist going forward.