Icarus does recognize many of the inertial delay elements (even in the VPI), but it does not really support them. Fixing this is likely a major rework of the scheduler and usually people do not need the functionality so I'm not giving this a high priority. If someone cannot work around this let us know and we will raise the priority. An example is in the test suite, but many more will need to be created to test all the different things that support inertial delays.
As a clarification. Inertial delays should work 100% for continuous assignments, gates and UDPs. Delays in a specify block have not been checked and Inertial delays from the VPI are know to not work. The specify path delays can be checked when we add $PATHPULSE, etc. support unless someone needs this earlier. The VPI interface may be the hardest to fix.
I just checked the specify block delays and added a test to the test suite and they work as expected, so only Inertial delays from the VPI are missing. Of course there are a number of issues related to the specify block delays that are missing as noted before.