E.g. I want to be able to filter for all open, in progress, or reopened issues in a specific project with summaries that do not contain the phrase ""Merge request".

According to the help files, you can search for entries where the text contains A but not B, but you simply cannot search for entries where the text does not contain B. All text exclusions require a text inclusion term before they work properly.

I find this very limiting and unnecessary.

There are many text search syntaxes, of which the most famous is regexp. Many of them allow such a search. it is sad that the Jira text search capability does not allow this. Perhaps the best solution is to allow regexp searches as an advanced function.