Positive news arrived for US citizens abroad seeking to travel back to the United States and unable to nab an appointment to renew a United States passport - especially for minors where an in-person appointment with both parents (or one parent and written permission of the other parent) has always been required. In a Media Note dated May 24, 2021, the US Department of State Spokeperson writes, “U.S. citizens currently overseas whose passports expired on or after January 1, 2020, may be able to use their expired U.S. passport for direct return travel to the United States until December 31, 2021.”
US citizens abroad have reported that they are not able to get appointments to renew passports. American consulates and embassies are short-staffed since the outset of the pandemic and are not providing enough calendar appointments. This has built up a large demand for passport renewal both for adults and children.
Although the US Department of State has historically preferred travel on an unexpired passport, the US Customs and Border Protection were comfortable taking an expired US passport as proof of US citizenship and a current state issued photo identity document as proof of identity, especially a foreign passport.
Now, it appears that the US Department of State has issued useful guidance, that they also will approve of travel to the United States on an expired US passport for citizens, through the end of 2021, if the passport expired around the outset of the pandemic - on or after January 1, 2020. The Media Note confirms that the US Customs and Border Protection is in agreement. US citizens traveling on expired passports that fit into this time frame would be wise to print and carry the Media Note when they travel to the United States to avoid hassle upon arrival and upon check in at their departure airport.
There are a few more restrictions on use of a recently expired US passport per the State Department website. 
1. Obviously, you must already have proof of US citizenship and the US passport itself. So, children born abroad may still need a Consular Report of Birth Abroad and US Passport. Hopefully, this new guidance will free up appointments for new parents to go through this procedure with their children.
2. Keep your travel route simple. You must be currently abroad and seeking to return directly to the United States. Travel to other countries on an expired US passport is not something on which the US Department of State is giving guidance - only travel to the United States. Transit, a short-term stopover, or connecting flight will not be an issue for the US State Department. Travelers may want to check with their airlines however.
3. The expired passport was a full validity passport. This means that it was originally valid for ten years, if you are an adult, and five years, for a child age 15 or younger when the passport was issued. Limited validity passports are typically issued for only one year, and would need to be reissued for travel to the United States per this Media Note.
4. Your passport is in good shape otherwise. It is not damaged or altered.
5. You passport is not lost or stolen. You cannot travel on a paper or digital copy of your passport, or on a US birth certificate and photo identity document under this Media Note.
6. You must be starting your journey from abroad to the United States. Those in the United States now are expected to avail themselves of the passport issuance services found there. The services there are not as backed up. In fact processing times for expedited services are about six weeks.
7. Special issuance United States passports such as diplomatic, official, service or no fee regular passports do not qualify for this guidance, and cannot be used to enter the United States if they are expired.