I was on a team that worked with a firmware vendor, from the US, for a bluetooth chip.
We would send in bug reports, they'd send us firmware with fixes. Except it was obvious they did not use source control because they would sometimes base patches off of old firmware versions that had the bugs they had fixed in newer versions. It was absolutely insane having to send emails like "hi, your latest patch is based on firmware from a year ago, can you please instead fix the firmware you sent us last month?"
I do understand why you might think that though. :-D
I wish I could put a label like this on all of my products and I've been wishing for this for over twenty years, but the reality on the ground is that our support ends when the support for the individual parts in our product ends. We've looked at our supply chain periodically to see if we can replace parts with better documented/supported comparable parts, but frequently there just really aren't any better options.
This is a great idea in concept, but I fear that the flaw in the FCC's proposed rulemaking is that only indirectly addresses the root cause (the software, documentation, and support/updates provided by chipmakers for their parts). Furthermore, by focusing on device manufacturers who are the weaker partners in the chain, the regulation is likely to punish smaller, more innovative manufacturers.
If there are no buyers then their attitude should change.
Since the specific "IoT device for the USA market" set of buyers is actually a small percentage of sales for most of the parts they sell, they really don't care to support their product from the IoT security perspective. This support is expensive, so it would very likely be cheaper for them to ignore the market completely.
Most of IoT is that way. We had sales cycles that were 2-3 years long and they would in the end buy 300 units. I then go back to my suppliers and say 'hey support these 500 ic's that you sold me for 10 years from right now' They would laugh me out of the room unless I am showing up with big bags of cash. That instantly makes the whole project unviable to sell/support.
It was a mess, but it may not be a good example because part of the confusion was that there was no newer firmware. Their firmware version was being reported in hexadecimal, but the latest firmware version was listed in decimal.
It is a problem of vendor locked in products where you have to buy a hub to do an update. If there even is an update. If you want to get a good picture of how sideways updating can even be watch the linus tech tips on where he wanted (and has the tech ability) to patch his light switches. But could not even get them to give him the correct firmware or even say if he could. Also many devices there is literally no way to even do the update. They flash it on the line and that is the last update it ever gets.
Also Supported and actually maintained in the hardware world can mean different things. So you will need to get your definitions up front correct. Supported could mean to a HW manufacture if the thing burns out ship a new one. The firmware is a secondary consideration.
Another aspect you will run into is licensing. I can sell a device but may not have access to the code. Example: The vendor who makes that code went EoL on their code 5 years ago. They will not even sell me the code as they may or may not even have anyone who works for them to give it away if they could. They may or may not want to sell me that software anymore as they have a new shiny they want to sell me. So I am stuck even though I want to update I can not do it. I had one vendor flat out refuse to give me the older docs because the item was EoL and they had a replacement product that cost like 5x. That was just to communicate with the thing. Not even to update it to a later revision.