In another comment, this user links a CRuby commit that they claim adds it. It seems legit.

Linked commit contains code for rotating tagged floats so bits 60..62 go to the least significant positions, and a comment about a range of unboxed floats between 1.7...e-77 and 1.7...e77, plus special casing 0.0

e.g. this excerpt:

    #if USE_FLONUM
    #define RUBY_BIT_ROTL(v, n) (((v) << (n)) | ((v) >> ((sizeof(v) * 8) - n)))
    #define RUBY_BIT_ROTR(v, n) (((v) >> (n)) | ((v) << ((sizeof(v) * 8) - n)))
    
    static inline double
    rb_float_value(VALUE v)
    {
      if (FLONUM_P(v)) {
 if (v == (VALUE)0x8000000000000002) {
     return 0.0;
 }
 else {
     union {
  double d;
  VALUE v;
     } t;

     VALUE b63 = (v >> 63);
     /* e: xx1... -> 011... */
     /*    xx0... -> 100... */
     /*      ^b63           */
     t.v = RUBY_BIT_ROTR(((b63 ^ 1) << 1) | b63 | (v & ~0x03), 3);
     return t.d;
 }
      }
      else {
 return ((struct RFloat *)v)->float_value;
      }
    }