Took me a bit to even remember what Texit was it had been so long since I'd heard it mentioned. Calexit was far more likely, although the Democrats running the state seem to be pissing off voters, so it seems less likely now.
However if a state did leave the US we would probably go back to the 49 star flag we (briefly) had after Alaska was admitted, but before Hawaii.
On a side note the "Today I Found Out" YouTube channel had an interesting video on the creation of the 49 & 50 star flags a few weeks (?) ago.