> I think PR https://github.com/python/cpython/pull/28674 has resolved this issue.

You're right.

_threadmodule.c now uses _PyDeadline_Init() which calls _PyTime_Add(), and _PyTime_Add() prevents integer overflows; Extract of its implementation:

// Compute t1 + t2. Clamp to [_PyTime_MIN; _PyTime_MAX] on overflow.
static inline int
pytime_add(_PyTime_t *t1, _PyTime_t t2)
{
    if (t2 > 0 && *t1 > _PyTime_MAX - t2) {
        *t1 = _PyTime_MAX;
        return -1;
    }
    else if (t2 < 0 && *t1 < _PyTime_MIN - t2) {
        *t1 = _PyTime_MIN;
        return -1;
    }
    else {
        *t1 += t2;
        return 0;
    }
}