What is your basis for a claim that 121.5 (guard) is unmonitored?
As a pilot I can say that claim is false - guard is very actively monitored everywhere in the United States. Were you referring to some other locations?
COSPAS-SARSAT no longer monitors 121.5, for various reasons, so no satellite systems regularly monitor 121.5. This means that 121.5-only ELTs are obsolete, and prohibited for installation by the FAA since 2018. Modern ELTs are COSPAS-SARSAT transponders like EPIRBs and PLBs with only minor aviation-specific features.
ATC does still monitor 121.5, but that's with an eye towards voice transmissions, not radio beacon activations. COSPAS-SARSAT has never carried voice traffic on 121.5, the satellites attempted onboard Doppler direction finding of the beacon tone (not very accurate at all, one of the reasons it is obsolete). At the same time, ATC no longer has RDF capability from most (all?) GATRs, so receiving the ELT beacon tone is mostly useless to ATC, and ATC is unlikely to receive it anyway since GATRs have very poor coverage down to ground. ELTs do still transmit on 121.5 for convenience of search aircraft, but it's becoming increasingly irrelevant with high COSPAS-SARSAT coverage (if the ELT activated at all, rescue coordinators already know the location by GPS coordinates) and increasing rarity of direction finding equipment (and pilot experience with RDF) on aircraft.
One way to sum it up is this: 121.5 is monitored for distress calls from aircraft in the air, but it is not monitored for distress calls from aircraft on the ground. The latter is the goal of search and rescue systems, and the use of 121.5 has been replaced by the much more modern COSPAS-SARSAT system originally developed for maritime rescue.
> The International Cospas-Sarsat Programme is a satellite-aided search and rescue (SAR) initiative. It is organized as a treaty-based, nonprofit, intergovernmental, humanitarian cooperative of 45 nations and agencies (see infobox). It is dedicated to detecting and locating emergency locator radio beacons activated by persons, aircraft or vessels in distress, and forwarding this alert information to authorities that can take action for rescue. Member countries operate a constellation of around 66 satellites orbiting the Earth which carry radio receivers capable of locating an emergency beacon anywhere on Earth transmitting on the Cospas-Sarsat frequency of 406 MHz.
It turns out that humans are all mutually interested in the general idea that we should rescue lost people. The reason for the name (half Russian, half English) is that this is an early peaceful purpose programme negotiated between the US and Russia. For a long time the only obvious reason to put things in space was so that you could kill the enemy, but COSPAS SARSAT is a system which needs space and doesn't kill anybody.
Helpful additional data on COSPAS-SARSAT, thank you! I didn't realize ELTs that only output on 121.5 were no longer a thing.
That said, the claim I was responding to was "121.5 and 243 MHz are no longer officially monitored for voice distress signals", which is pretty obviously false.
As a pilot I can say that claim is false - guard is very actively monitored everywhere in the United States. Were you referring to some other locations?