In some mail from Fernando Gont, sie said:
The IPv4 minimum MTU is 68, and not 576. If you blindly send packets
than 68 with the DF bit set, in the case there's an intermmediate with an
MTU lower that 576, the connection will stall.
And I think you can safely say that if you see any packets trying to
indicate that the MTU of a link is "68" then you should ignore it.