This has been discussed before, probably more than once.
Automatic IDs and shortcut links are implemented in several implementations and CM doesn’t forbid such extensions, I think. Assigning (or at least using) explicit IDs, however, could be done in a more indirect way than you show:
* [This Is a Header]
* [This Is a Subhead][subhead]
# This Is a Header
## This Is a Subhead
[subhead]: [This Is a Subhead]