Relevant code

• decoder.go, L61:

  } else if b < 0x20 {
    // skip byte introduced by DCE
  }

• encoder.go, L65

  if (b) < 0x20 || ((b)&0x7f) == 0x7d || ((b)&0x7f) == 0x7e {
    // ...
  }

Relevant Documentation

• RFC 1662, section 4.2 Transparency:

  After FCS computation, the transmitter examines the entire frame
  between the two Flag Sequences. Each Flag Sequence, Control Escape
  octet, and any octet which is flagged in the sending Async-Control-
  Character-Map (ACCM), is replaced by a two octet sequence consisting
  of the Control Escape octet followed by the original octet
  exclusive-or'd with hexadecimal 0x20.

  On reception, prior to FCS computation, each octet with value less
  than hexadecimal 0x20 is checked. If it is flagged in the receiving
  ACCM, it is simply removed (it may have been inserted by intervening
  data communications equipment). Each Control Escape octet is also
  removed, and the following octet is exclusive-or'd with hexadecimal
  0x20, unless it is the Flag Sequence (which aborts a frame).

• pppd man page:

  If no asyncmap option is given, the default is zero, so pppd will ask the peer not to escape any control characters.

Add decoding of flags when the frame shifted n bits. There are situations when a frame is not a multiple of a byte and the flag is shifted by several bits.
like this:
[]byte{0x3F, 0x81, 0x84, 0x00, 0xF8, 0xE0, 0x6B, 0xFD, 0xFC}
0x3F and 0xFC - HDLC Flags