I find it hard to believe that anyone's just going out and hiking 3,000 miles without going into resupply towns, using public laundromats, local grocery stores, staying at hotels/hostels, and hitching rides into the communities. All of the activities involved in resupply put the hiker in risk of contacting Covid-19, and then they carry that to the next trail town and to other hikers.
Let's be clear; it's not the hiking part that's dangerous about thru hiking this year. It's the part where they have to interact with the public.
(This comes from someone with a lot of thru hikes in his past and was planning a thru hike this year but canceled plans, even though I wish I didn't have to. The health of the greater public is at stake here. It's not just about the individual, and thru hiking isn't just about walking 3,000 miles, alone in the woods. I probably interacted with more people during my 2019 CDT thru hike than I did for the rest of the year off the trail!)