Groundhogs - Earth Shanty (1972)
www.youtube.com

When Did FM Radio First Become Available in Cars?
earthrepairradio.com

Was there ever such a AM/FM mandate for non-vehicle radios, i.e., those in homes?

I cannot find one.

There was the non-duplication rule, though ...

FM Non-Duplication Rule
en.wikipedia.org

... The FM Non-Duplication Rule was adopted by the U.S. Federal Communications Commission (FCC) on July 1, 1964, after a year's consideration.

It limited holders of FM licenses in cities of more than 100,000 who also held AM licenses to simulcasting no more than 50 percent of their AM signal on the FM station.

The commissioners considered the excessive simulcasting wasteful and an impediment to the development of FM broadcasting.

A year later, the FCC reaffirmed the rule, and, after a delay requested by broadcasters, set its effective date for October 1965; some stations were granted exemptions if they could demonstrate their simulcasting served the public good. ...

As I was growing up, we always had an AM/FM radio in the house. I remember listening to WABC in NYC on their FM channel, 95.5.

That non-duplication rule, though, gave rise to FM stations that would broadcast songs longer than the three minute limit of the AM stations.

Can you say WNEW-FM (102.7)? I knew you could.

WFMU
en.wikipedia.org

... WFMU (91.1 MHz) is a non-commercial, listener-supported, independent community radio station licensed to East Orange, New Jersey, with studios in Jersey City. It is owned by Auricle Communications, broadcasting a free-form radio format. ...

I receive the station over-the-air here.

Yeah, "free-form" is a proper appellation for the station.

My guess is that they also have a streaming link ...

WBMX
en.wikipedia.org

... WBMX (104.1 FM) is a radio station in Boston, Massachusetts, United States, serving Greater Boston with a hot adult contemporary format. The format started at 98.5 FM on February 9, 1991, and moved to 104.1 FM, replacing WBCN on August 12, 2009, to allow for the launch of WBZ-FM at 98.5 the next day. Its studios are located in Brighton, and its transmitter is on the upper FM mast of the Prudential Tower in Downtown Boston.

The 104.1 MHz facility went on the air in 1958 as WBCN.

A classical music station for its first ten years, beginning in 1968 WBCN featured a rock format for 41 years.

Known as "The Rock of Boston", WBCN became a legend in the rock music industry for breaking many bands, including U2.[3] ...

Clear-channel station
en.wikipedia.org

... A clear-channel station is a North American AM radio station that has the highest level of protection from interference from other stations, particularly from nighttime skywave signals. This classification exists to ensure the viability of cross-country or cross-continent radio service enforced through a series of treaties and statutory laws. Known as Class A stations since the 1983 adoption of the Regional Agreement for the Medium Frequency Broadcasting Service in Region 2 (Rio Agreement), they are occasionally still referred to by their former classifications of Class I-A (the highest classification), Class I-B (the next highest class), or Class I-N (for stations in Alaska too far away to cause interference to the primary clear-channel stations in the lower 48 states). The term "clear-channel" is used most often in the context of North America and the Caribbean, where the concept originated.

Since 1941, these stations have been required to maintain a transmitter power output of at least 10,000 watts to retain their status. Nearly all such stations in the United States, Canada and The Bahamas broadcast with 50,000 watts, with several clear-channel stations in Mexico going as high as 150,000 watts, and XEW in Mexico City having formerly operated with 250,000 watts for over 80 years before moving the transmitter and reducing to 100,000 watts in 2016. Cuba was originally included in the plan and had several stations given clear-channel status, but stopped participating after the Cuban Revolution of 1959. ...

Fun stuff ...