`spriteframe_t` is defined as

  typedef struct
  {
    // If false use 0 for any position.
    // Note: as eight entries are available,
    //  we might as well insert the same name eight times.
    boolean rotate;

    // Lump to use for view angles 0-7.
    short lump[8];

    // Flip bit (1 = flip) to use for view angles 0-7.
    byte flip[8];
    
  } spriteframe_t;

So yeah, on C99 and C++ this always had UB and could've broken at any time – though I presume compiler devs were not particularly eager to make it ill-behaved just because. But in pre-C99 it's entirely fine, and `rotate == true || rotate == false` could easily be false without UB.

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(*) other than `char` for which setting the MSB is… not UB but also not the best idea in general.

C99 does not use bool unless you explicitly include stdbool.h, which won't be in any pre-C99 code.