Hi Gary,
Upon further checking, I can confirm that my "Removed" status from the UI's test account is just equivalent to "Excluded from ad group" status. Actually, when I checked the age ranges again that I used previously, they are still in "Removed" status. When I manually changed one age range with "Removed" status to "Exclude from ad group" using the new UI, all the other three age ranges with "Removed" status was automatically changed to "Excluded from ad group" (see my attached screenshot). I am just not 100% sure on why the new UI for my test account showed the "Removed" status originally. However, I can confirm that they are just the same as there are only two options in the new UI for the age ranges, to "Enable" it or to "Exclude from ad group".
So, in regards to your screenshot, that is correct. You successfully excluded the two age ranges in the API and it reflected in the UI. However, you just need to use the ADD operator, and not the REMOVE or SET operator as it will generate an error just like your testing.
Hope this helps.