This sounds like it is more down to your monitor than the Pi.
The EDID should be readable even when the display is powered off - there's even a 5V line up the HDMI cable to power the EDID EEPROM. Testing on my Dell monitor here I can read the EDID happily when it is turned off via the soft power button (not when physically unplugged from the mains).
The Pi has never supported full hotplug. It supports more display outputs than it can drive simultaneously, therefore detects what is connected at boot.
With FKMS (and eventually full KMS) it already reads the EDID on demand. We can probably plumb in the connector status to allow monitor switching once booted. Whether the change is also made to make the assumption that if no other displays are attached that HDMI should be active and wait for a hotplug connect status is a different question.
Software Engineer at Raspberry Pi Trading. Views expressed are still personal views.
I'm not interested in doing contracts for bespoke functionality - please don't ask.