My key letter for that search engine is a small "s"

I'm not sure why you're having a problem, as Start page, using the key letter is working for me from the Opera omni search/address bar.

On your question, I don't know the answer, but it's been quite a few updates and no default engines have so far been added, that I'm aware of.

Btw, I just added startpage to the search engines and assigned the letter S with startpage. I did enter the letter S in the search bar incl a space and then my search query, unfortunately it did not work but it went straight to the website it self.

I was wondering if you know if this default search engine feature will come out on the next Opera versions?

https://startpage.com/

and right click on the search engine there, you can add the search engine to Opera. It will assign/or you can select a keyletter(letters), and then you can search from main Opera search bar, by just typing that letter [a space] and then your search query. It's quite simple to use startpage.com in Opera 25 for your search. All you have to do is start your search with that letter (s). Opera limited the default search engines to prevent malware hijacking users and taking them to unsafe websites.