We could look into bringing the score field back. I wasn't around when it was deprecated so can't say why exactly it was removed. According to the git history most search backends used by mediawiki don't have a score that can be exposed, and those that do use very arbitrary scores. For example whenever we change how queries are built the scores change, but those changes have no bearing other than their use in relation to other results for the same query.
For example the best score for https://www.mediawiki.org/wiki/?search=developer+summit&fulltext=1&cirrusDumpResult is 638, but the best score for https://www.mediawiki.org/wiki/?search=developer+summit+mediawiki&fulltext=1&cirrusDumpResult is 358. That certainly doesn't mean the top result for the first query is twice as good as the top result for the second query, it's just arbitrary. As another example, we changed how we build search queries abotu 6 months ago and the top score for the first query dropped from 1057 to the current 638. The resulting top result (in this case) is exactly the same, but the score dropped. This isn't an indication the result is worse, it's just an arbitrary number.