If you mean "hot staging" they've done that on most of the flights so far I think.
They said something about removing a protective layer. Just a little worried, you know?
If they've done it once they'll have done it for all of them (that were staged). To over-explain: it takes a second or three for the tubopumps (ie most of an engine) to spin/heat up and hit full thrust. And if you're not at full thrust, or at least above what the booster stage was just putting out, you get fuel ullage. Most propellants but cryofuels especially always have a gas bubble because liquid oxygen/hydrogen/methane/lawnmower fuel/whatever don't really want to be liquid. It's like tapping the breaks all of a sudden mid-staging, so the gas bubble flies to the bottom where you're trying to pump it from and the engines stall.
So virtually everything hotstages and always has. The main exceptions are ancient pressure-fed designs but those are shitty and need heavier tanks. In microgravity you have the same issue (at best, bubbles in random places) but you only need a tiny amount of inertia so that's resolved by tapping RCS thrusters or dedicated ullage thrusters for a sec. Or especially in the past, little mini solid rockets.
And you
can do similar shit during ascent too but it's way less common (the Falcon 9 does it though), I think it depends on your ascent profile, like if you actually need to coast for a while you might take the opportunity to not damage the paint job, but burning all the way up is more efficient.
Might sound scarier with SpaceX rockets because they reserve some propellant in the tanks for soft descents, but remember that's the same super-cooled propellant you use to constantly cool the engines that are putting out the exact same face-blasting fire. Actually, less, because the booster stages blast way the fuck more face than upper ones as a rule. And it's only for a second (or 1.1 seconds for the vacuum raptors apparently). So it's fine. Totally conventional.
