or is it effective all the way back at I-140 time where people would then need to spend years away from the US?

On visa forums this method is commonly discussed. By entering on an ESTA/B-2 with the intent to marry a US Citizen, they're committing immigration fraud, inherently. You would be denied entry at the border if you admitted to your plans.

The correct way to do this is to file a K-1 visa outside the United States, or marry outside then file a IR-1/CR-1.

I read that it used to not be like this, that it used to be possible to renew the _visa_ itself from inside the US, but that got changed before my time. I can only imagine that the reason for that was that non-citizens inside the US are entitled to due process, but non-citizens outside the US are not. And denying a visa to somebody outside the US is therefore a lot easier than denying it to somebody inside the US, and essentially cannot be appealed.

When I applied for AOS form H1B to Green Card, I didn't have to leave the US. With this change, I would have had to. The only reason I can think for this change is that denials of AOS would now become unappealable. I hate this.

F1 and h1 are non-immigrant visa.

American law only allows a person to reside in the country with one Visa type.

The green card is an immigrant visa - and the new visa is issued through an adjustment of status for those inside the USA (backlogged) or by consulates (nearly immediately).

So this is a good thing. It’s easy to get alarmed.

I wonder how this would work with a K-1 "Fiancé" Visa. Typically a K-1 holder can enter the country as long as they get married within 90 days, and then the family stays together while the I-485 is processed. Now what? Come to the USA, marry the US Citizen, and then you're banished back to your home country?

There's also the K-3 which lets the foreign spouse enter as a non-immigrant to keep the family together while the I-485 is processed. Are they getting rid of that entirely?

This is all totally bonkers, likely not well thought out, and pretty cruel to families, which is completely on-point for this Administration.

I did consular processing when I got my Green Card. It's the FINAL step fo the GC process. You don't need to be outside the US for all the other stages, in fact I think if you leave during some parts, it would be considered abandoning your application. It just means that while you're in the US, you need to schedule an appointment at the US embassy/consulate in your home country, and fly back. Then you go through the appointment and there on the spot you're approved or rejected. It's a big nerve wracking but unless you lied you will be fine. Then you fly back to the US.

For me CP was much much faster, on the order of months.