While we can choose which field to order by when searching in advanced mode:
… we cannot choose whether we want to order ascending or descending.
Adding orderdesc modifier would easily make it possible:
order:likes is already descending, right?
So what you want is “the opposite of the normal order that this term means”.
Not sure about this, but… How about order:-views to start at 0 and go up, while order:-latest starts at the oldest?
struggling to think of use cases for orderdesc:likes or orderdesc:views, though I am open to possibly adding order:earliest (its quite rare to need though)
We should find a way to allow for opposite ordering for any ordering type (kind of a universal way of doing it), then easier to remember and use.
You never know what one could come up with 
I’d use orderdesk:likes to identify which type of material is the worst in terms of people likes and not create similar topics, or not include them in the lists.
orderdesc:views might possibly indicate topics that need some more useful information to get more attractions from organic search.
I think there might be more use-cases I can’t think of, why not just make it possible with a single operator.
I want this feature in order to support an “inbox” use case for processing forum traffic: I have a search that I want to sort by least recently posted (so that I can work down the queue of posts awaiting a response), and I can only get most recently posted (afaict).
You might want to consider using the Unanswered filter: /latest?max_posts=1.
However, I did also write a PR to add this sorting option:
Taking suggestions as to what the name in the dropdown menu should be, I don’t really like “Oldest Update”.
I was looking for a way to sort results by oldest (ascending chronological order, adding these terms for future search on meta…) when searching inside a topic.
I couldn’t find “Sort by Oldest Post” (there’s only Latest Post), but I did try “Sort by Latest Topic” and it did work.
I think I understand the logic, but the words are a bit counterintuitive in this particular case.
A choice of ascending/descending chronological order would be, for me, a nice addition, I sometimes try to do that, and did it on other forums that allowed it. Often most for fun than looking for specific and interesting info though.
Also, I wonder how often people sort by Latest, compared to the default “Relevance”. I often use “by Latest” on many forums.