This is a bit field of a single bit (the : 1 means one bit, not initialize to true). So generally 32 of these variables in a given class/struct has the same memory footprint as 1 of these variables in a given class C Bit Fields | Microsoft Learn
As to why, I am not sure. On different platforms, could be a combination of byte alignment / performance / possible memory savings (bool being implemented as an int by the compiler) / personal preference, or even legacy UE4 reasons.