You definitely don't as long as the site supports OpenSearch - the link they supply even mentions this. Maybe the engines that were removed don't support it?
Yeah, it's weird, their linked support article says you can add any website with opensearch but that's not mentioned in this one. I'd be surprised if search engines that were available before this update either didn't support opensearch or broke their Firefox integration so hard that they needed to be removed. The way the article is worded makes me think this is not a technical issue, though.
Came here to comment about the same. It's odd that this KB mentions extensions and not OpenSearch. In fact, extensions are always a worse choice from privacy perspective.
might be related to this:
"Mozilla intends to deprecate OpenSearch and eventually remove it from Firefox."