It doesn’t work very well in iOS 13, either. Many times, I’ll type the exact text that appears in the app, and it won’t find it. So I mostly just scroll and read. A long time ago, I reverse-engineered the transcript file format for EagleFiler, so that’s how I search older messages. Unlike in Messages, the matches are highlighted, and you can select the text.
When I need to find something in my iMessages I just grab my iPhone. And let’s not forget that this option has only become viable since iOS13, before which it was just as bad on there as well.
That’s what I do as well. Search works so much better on the iPhone. Keying in the same text generally gives me no results on the Mac and what I’m looking for on the iPhone.
In comparison, It’s so easy to search in Telegram and WeChat on macOS.