There are a lot of format units supported in PyArg_Parse* functions, but some of them are rarely or never used in current CPython code. Some of format units are legacy from Python 2 and are not needed in modern Python 3 code or can be replaced with custom converter.

Here are results of grepping (not including Modules/_testcapimodule.c).

"es", "es#", "et#", "z*", "Z#" are not used.

"y#":
Modules/_io/textio.c:2334:        if (!PyArg_ParseTuple(_state, "y#i", &dec_buffer, &dec_buffer_len, &dec_flags)) { \

"z#":
Modules/_ctypes/_ctypes.c:3327:    if (!PyArg_ParseTuple(args, "is|Oz#", &index, &name, &paramflags, &iid, &iid_len))

"u#":
Modules/arraymodule.c:248:    if (!PyArg_Parse(v, "u#;array item must be unicode character", &p, &len))
PC/winreg.c:1547:    if (!PyArg_ParseTuple(args, "OZiu#:SetValue",

"y":
Modules/_io/textio.c:2334:        if (!PyArg_ParseTuple(_state, "y#i", &dec_buffer, &dec_buffer_len, &dec_flags)) { \
Modules/_cursesmodule.c:2790:    if (!PyArg_ParseTuple(args,"y;str", &str))
Modules/_cursesmodule.c:3026:    if (!PyArg_ParseTuple(args, "y|iiiiiiiii:tparm",
Modules/posixmodule.c:3767:    if (!PyArg_ParseTuple (args, "y:_getfullpathname",
Modules/posixmodule.c:3872:    if (!PyArg_ParseTuple(args, "y:_isdir", &path))
Modules/faulthandler.c:941:    if (!PyArg_ParseTuple(args, "y:fatal_error", &message))

"et":
Modules/socketmodule.c:4499:    if (!PyArg_ParseTuple(args, "et:gethostbyname", "idna", &name))
Modules/socketmodule.c:4667:    if (!PyArg_ParseTuple(args, "et:gethostbyname_ex", "idna", &name))
Modules/socketmodule.c:4744:    if (!PyArg_ParseTuple(args, "et:gethostbyaddr", "idna", &ip_num))
Modules/_tkinter.c:2099:    if (!PyArg_ParseTuple(args, "et:splitlist", "utf-8", &list))
Modules/_tkinter.c:2162:    if (!PyArg_ParseTuple(args, "et:split", "utf-8", &list))
Modules/_ssl.c:3038:        if (!PyArg_ParseTupleAndKeywords(args, kwds, "O!iet:_wrap_socket", kwlist,
Modules/_ssl.c:3070:        if (!PyArg_Parse(hostname_obj, "et", "idna", &hostname))

"s*":
Modules/_codecsmodule.c:188:    if (!PyArg_ParseTuple(args, "s*|z:escape_decode",
Modules/_codecsmodule.c:552:    if (!PyArg_ParseTuple(args, "s*|z:unicode_escape_decode",
Modules/_codecsmodule.c:569:    if (!PyArg_ParseTuple(args, "s*|z:raw_unicode_escape_decode",
Modules/_codecsmodule.c:696:    if (!PyArg_ParseTuple(args, "s*|z:readbuffer_encode",
Modules/_ssl.c:3734:    if (!PyArg_ParseTuple(args, "s*d:RAND_add", &view, &entropy))
Modules/fcntlmodule.c:225:        if (PyArg_Parse(ob_arg, "s*:ioctl", &pstr)) {
Modules/clinic/arraymodule.c.h:278:    if (!PyArg_Parse(arg, "s*:fromstring", &buffer))

"s#":
Modules/_gdbmmodule.c:128:    if (!PyArg_Parse(key, "s#", &krec.dptr, &krec.dsize) )
Modules/_gdbmmodule.c:176:    if (!PyArg_Parse(v, "s#", &krec.dptr, &krec.dsize) ) {
Modules/_gdbmmodule.c:194:        if (!PyArg_Parse(w, "s#", &drec.dptr, &drec.dsize)) {
Modules/fcntlmodule.c:71:        if (PyArg_Parse(arg, "s#", &str, &len)) {
Modules/_ctypes/_ctypes.c:2569:    if (!PyArg_ParseTuple(args, "Os#", &dict, &data, &len))
Modules/clinic/unicodedata.c.h:361:    if (!PyArg_Parse(arg, "s#:lookup", &name, &name_length))
Modules/clinic/_dbmmodule.c.h:62:    if (!PyArg_ParseTuple(args, "s#|O:get",
Modules/clinic/_dbmmodule.c.h:95:    if (!PyArg_ParseTuple(args, "s#|O:setdefault",
Modules/clinic/_gdbmmodule.c.h:150:    if (!PyArg_Parse(arg, "s#:nextkey", &key, &key_length))
Modules/_dbmmodule.c:108:    if (!PyArg_Parse(key, "s#", &krec.dptr, &tmp_size) )
Modules/_dbmmodule.c:132:    if ( !PyArg_Parse(v, "s#", &krec.dptr, &tmp_size) ) {
Modules/_dbmmodule.c:150:        if ( !PyArg_Parse(w, "s#", &drec.dptr, &tmp_size) ) {
Modules/_dbmmodule.c:336:        if ( !PyArg_Parse(default_value, "s#", &val.dptr, &tmp_size) ) {

In future may be we could deprecate some format units and remove them in 4.0.

This issue is a meta issue. Every case should be considered individually.