Re: the ESD diode
The ESD diode to VSS is meant for Open-drain I/O only, found in older VLSI circuits in order to save space. Now it is very rare to use single ESD diode. With today's advanced nodes from 180 down to 65, all I/Os are using ESD diodes to VDD and VSS so as to provide adequate protection to CMOS circuits which are very sensitive static charges. Sometimes series resistors are added to the I/O for current-limiting, especially in Open-Collector Bipolar, but was an old method and hardly used today.