What is the value for the cgroup's limit_in_bytes if the memory is not restricted/unlimited?

The value comes from the cgroup setup in the memory management layer; by default, it’s set to PAGE_COUNTER_MAX, which is LONG_MAX / PAGE_SIZE on 64-bit platforms, and multiplied by PAGE_SIZE again when read.

This confirms ilkkachu’s explanation: the value is the maximum 64-bit signed integer, rounded to the nearest page (by dropping the last bits).

That's the highest positive signed 64-bit integer (2⁶³-1), rounded down to multiples of 4096 (2¹²), the most common page size on x86 systems. It would seem difficult to get anything higher while avoiding possible confusion between signed and unsigned, so it seems at least a reasonable approximation for infinity.

That said, I don't know for sure, this is just a guess.