I found this support forum post but the solution provided doesn’t work.
Edit (thanks to @moreeni@lemm.ee for the Stack Exchange link):
Solution - Go to Settings -> Home -> Enable Shortcuts -> Go to a new tab -> Google will be pinned by default, hover over the Google icon -> Click on the 3 dots menu -> Unpin
You can add ecosia without the extension, just open the website, click the url bar and it’ll be among the search engines with a plus on top. Clicking it adds it