When I've been after old comments of mine or others and know some text from it filtering been straightforward. Results can be sorted in chronologically ascending order and date ranges can be set in addition to the standard filter options.
It's not perfect though. There's no exclusion filter option, some queries are automatically broadened without a way to enforce strict string matching (eg: searching enemies will also include results for enemy but wrapping in double quotes like "enemies" doesn't prevent this), the pagination when there are too many results for a query maxes out at ~200 pages even if more pages are shown/should be accessible via the navigation (seems like a bug), the pagination is also only at the bottom which adds a little extra time navigating and attachment filenames can't be searched.
That said, even with the above caveats I manage to find what I'm after reasonably quickly the vast majority of the time, even things years old.
My friends and I have had a server since ~2016 that we use and I've been able to find whatever I've needed even back that far. It might be harder in a higher volume server (we have about 20 actives and another 20 semi-active chatters).
But the search will let you specify time periods, channels, who sent the message, etc. It also gives a line of context around the result, which takes up some space, but I find is quite useful for, well, getting the context of the statement and figuring out what it meant.
I think the most important facet for (very) old content searching, though, is that you can almost immediately go back in the chat history to any result. Whereas other chat apps make that slow or difficult.
Since my discord activity is growing by the month, is search good/great with (very) old content?