Similar to how you can favorite trades by clicking the >> arrow (which add a gear icon to it), perhaps you could right click to instead add a red Ø icon and hide the trade? It would place them at the end of the list when visible, and also include a "Exclude hidden" on the search filters

The use case for this is early game, pre rocket; trades with items you haven't researched on the buy side (right) are easily filtered, because they have -500% or more productivity; currently I have items like barreled fluids or circuity or concrete variants on the sell side (left) but I have no easy way to trade/produce for them yet and they're in the way of other potentially valid trades

I like what it's trying to solve, but it seems like it would end up being a chore to hide the ones you don't like, or to even have to think about which trades you'd want to hide. Like you said, it might be useful for early game, but it definitely would become a pointless feature once you get hundreds or thousands of trades in your save.

Do you think something more fundamental could change that benefits all stages of the game regarding promoting trade relevance?

My use case for this was because I wanted to filter away inputs that I have not researched, since I wouldn't have a ready supply of the item to actually do that trade more than once