I only know that in the Spice the breakdown voltage of a diode is "BV". I probably can assume that you mean the breakdown voltage from the source of one of the mosfets, right? There is a parameter in the spice files of the SKY130 Transistors thats called "bvs". I think that means that s stands for source. e.g. sky130_fd_pr__nfet_01v8.pm3.spice on line 240: + bvs = 11.7 I hope that answers your question...

Indeed, your response answered my question. Thanks a lot!