And yes, you guessed it, it was when Cap was found worthy to wield Mjölnir, Thor’s hammer, during the final battle with Thanos.
Remember in the first Thor movie, Odin said that whoever is deemed worthy will have the power of Thor.
We saw this with the flashes of lightning, Thor’s ability, even without use of the hammer.
So what if this also means that Cap in now immortal? He could be in the MCU for a lot longer.