That main stage sure looks like it got quite hot, or chard on reentry.
Was anything said about if they can re-use this again?
Yeah, so did the two other GTO profile flights that successfully landed the boosters while also doing those risky 3-engine "suicide burns" for landing. Those did not have enough fuel to do a normal re-entry burn so they had very hot re-entries. Those may never fly again. So when I realized this mission was going to be the same kind of thing, I figured it would be retired if it survived. Musk said he is donating it to KSC. So, officially that was its last flight.
So, here's a pic of the upper section grid fins and interstage, by Scott Murray:
The grid fins are aluminum, with an ablative coating. But some of the hot re-entries like this, the aluminum itself started to burn. For the next Falcon 9 version, "Block 5", it will use titanium forged grid fins. The hypersonic airflow past and above the grid fins sure caused some extreme heating charred splotches on the interstage. It is made of carbon fiber and aluminum, and certainly the RESIN that bonds carbon fiber to itself and the CF to the aluminum is probably the weakest link, potentially de-bonding or bubbling (not sure of how much heat CF itself can take, it varies widely depending on how it was fabricated). I do not know if that charring is simply ablative paint, or if the carbon fiber/aluminum structure was damaged (or weakened in any way) by that heat. I also do not know that if it were structurally damaged, if it would be practical to replace damaged/weakened interstages with new ones while being able to re-use most of the rest of a booster.
They have said that Block-5 should be able to make more flights, and need less refurbishment work. So perhaps they will also be protecting the interstage better, or make it more practical to replace them. Since the area affected upstream is not too big, and if the current method is allowing heat damage, I could see them adding either extra-thick ablative coatings in those key areas, or some other sacrificial coverings, to protect the CF/Aluminum structure (leftover charred ablative could be "sandblasted" clean using fine abrasive that would not hurt the structure. They are probably doing that anyway with anything covered with ablative).
I did notice during re-entry, when the grid fins started to glow orange, just before the camera got obscured, that the "paint" was bubbling on the interstage below the camera. But that may have been a thin ablative coating.