For long haul international, here's a very premature speculation of what will stay and go.
I don't think UA will cut any South American cities since most cities are only served through IAH in the network. We will probably lose MUC as Star traffic consolidates to FRA, and AMS will get downgauged due to reduced O&G premium demand. I see SYD getting cut and connections funneled through SFO/LAX.
BA - mostly safe
AF - this has been one of AF's weakest routes. If any Euro carrier leaves Houston, my bet is on AF.
KL - safe since Shell is huge in Houston, and this has long been one of their most profitable routes
LH - nothing will change. IAH-FRA is a Star trunk route for Houston-Europe or Germany-Latin America connections.
EK/QR - will not get cut because ethnic traffic to South Asia remains strong. EK will likely stay 777 year round.
ET - this is a young route with poor aircraft utilization, so it's probably a goner.
NH - ANA has not cut a single international destination since expansion, and this one's staying to give passengers a choice between HND or NRT.
BR - This route has been hugely successful with the Vietnamese diaspora who will continue to travel after the pandemic
CA - This is another route driven by ethnic traffic. Don't see this one being dropped unless US-China relations go from bad to worse
SQ - IAH-MAN traffic will suffer if oil stays low. There are too many IAH-SIN one-stops, so I don't see this one staying.
NZ - If it stays, it will be at a reduced frequency
I doubt SQ will pull out of IAH, SQ was been benefiting with the new MAN stop over plus the A350, also the flight is 5th freedom. Whenever SQ operated via DME to IAH, SQ weakest load factor to North America was IAH. Though since front cabin, and cargo kept the flight alive though with such low load factors. Almost every carrier they came to IAH has always mentioned about how fast they can get you to SIN, airlines like KE, NH, CA, and BR all boasted about how fast they can get you to Southeast Asia. Though SQ is still in the game despite all those carriers targeting that region of the World. When the Covid panic started, SQ were decreasing flights to LHR, LAX, SFO, JFK, and other stations that properly generated more revenue, and higher load factors then IAH. Though IAH never got axed until the end.
AMS i doubt will be decreased from SQ or UA, due to also cargo demand on this route. CDG i can see being decreased by frequency, IAH is important to AF. EK and QR will remain daily flight, EK will probably go to 777 though who knows. I can see TK going down to 4 to 5 weekly, as they normal do during the low season. LH i can see sending the 748. MUC was already seasonal, and I can see it remaining like that. UA to SYD depends on the competition with AA/QF at DFW, as we see UA/NZ and AA/QF battling on routes to the Oceania. I can see SYD going seasonal.
Though we are probably gunna cuts in spending from Airlines offering amenities from on board, to the lounges.