I called my chimney company and they told me all they'd heard of was people building faux chimneys for the little fellows to nest in. On the other hand, they also told me they were besieged with calls to put new caps on chimneys for people who had problems with swifts and their mess & noise. Sounds like the problem will get worse before it gets better.
I love watching them out at my folks' house. They don't use their chimney so they let them nest in it every year.
As for me, I had no cap on my chimney in my old home for 15 years and had birds fall down in the bottom every year. Never swifts but always other types of birds like grackles and starlings. It was much more traumatic for them than us. Luckily it only happened in the summer months so we had no unexpected roasted bird problems.
My newer home has a cap on the chimney so no birds and no furry critters, but the doggone water bugs are coming in right now with the drought and the heat.
We cheer the dog & cats and their 'hunt" on a nightly basis, unfortunately. They all hunker down and stare at the fireplace screen until it's time to pounce. One cat even goes IN the fireplace to get them. Great sport for them but we always wonder if any get away. So far we haven't had any unpleasant close encounters. Let's hope it stays that way!
Good luck and I'll pitch in if I get any more information that will help.