Please see Nano screenshot below. This depicts two consequtive serial bursts of data transmitted to a DAC along a Two Wire Interface which, for the purposes of your question, is not too unlike I2C.
The data is 16 bits in length and as you can see is transmitted in about 20 microseconds. I make that pretty close to 800 kbit/s.
Can you see all the bits or work out the value being transmitted? No. Can I see that the bits are well formed? No.
However, I can see that the burst is there. This is good enough for my purposes, your milage may vary. Of course, at 400 kbps you are running about half this rate, so you will see a little more detail. At 100 kbps, you will see a bit more detail still.
So the answer depends on how good is good enough.