Julia has unsafe operations and undefined behaviour, but has not really defined what is safe and what isn’t. The meaning of unsafety and undefined behaviour is mostly (only?) known to a handful of core devs.
However, a recent PR explicitly mentions that creating Bool values with other bitpatterns than 0x00 and 0x01 to be undefined behaviour: Add devdocs on UB by Keno · Pull Request #54099 · JuliaLang/julia · GitHub