"t#" format was introduced by r11803 (11 years ago): "Implement new format character 't#'. This is like s#, accepting an object that implements the buffer interface, but requires a buffer that contains 8-bit character data."

Python3 now has a strict separation between byte string (bytes and bytearray types) and unicode string (str), and has PyBuffer and PyCapsule APIs. "t#" format can be replaced by "y#" or "y*".

Extract of getarg.c:

      /*TEO: This can be eliminated --- here only for backward
        compatibility */
    case 't': { /* 8-bit character buffer, read-only access */

In Python, the last function using "t#" is _codecs.charbuffer_encode() and I proposed to remove this function in #8838. We can also patch this function.

I don't know if third party modules use this format or not. I don't know if it can be just removed or if it should raise a deprecation warning (but who will notice such warning since there are disabled by default?).