I'm adding Kalitta due to whats being talked about on the K4 thread. I suspect the delays for the next class date is Covid-19 driven. Pal at new Eastern just said new hire classes postponed.
Legacy:
American
Delta
United
Major & 121 Charter:
Frontier
Eastern
Cargo:
Kalitta Air (Unofficial - No new hire class dates scheduled as of now (Confirmed), rumor next interview group will be postponed as well.)
135 & 91k:
Flexjet