Probably nothing nuclear. Even if you didn't care about getting nuked in return, irradiating the waters around the Taiwan strait is just going to make any armed reunification even more of a pain in the ass.
My guess is that the PLA just kept mobilizing more and more conventional forces until the US finally felt threatened enough to call it off. The PLAAF's mobilization is the most obvious since they went on to fly around Taiwan right afterwards, but I wouldn't be surprised if it turns out significant amounts of PLARF and PLAN assets were also ready to go.
I think the PLA just mobilised sufficient forces for armed reunification and the US realised that if they triggered a shooting war by going through the Taiwan strait, the first thing that will happen is their forward deployed carrier gets obliterated, the second to closely follow is the successful armed reunification before they can get a replacement carrier into the AO.
It was a stupid move to use a carrier to try to call China’s bluff on this, since loosing that carrier as the opening move would put the US in an unrecoverable position and by the time they can muster enough forces for a credible stab at preventing armed reunification, the mission would have creeped into launching their own amphibious invasion of a PLA liberated Taiwan.
They will probably have another go at salami slicing this new red line, but with probably a Burke, while a couple of CSGs observe from a safe distance.
But in the meantime, it’s kinda funny that American salami slicing against China has materially weakened American positions instead of China’s. But I guess that’s par for the course for America and the west these days.