Thresh will get TT lore lines at the next TT lore recording which will also give Evelyn lore lines. This will be at the point where we are ready to reveal more about Evelyn's connection to the shadow isles.
We are ready to do lines for Thresh now, but we don't do recordings for 3-4 lines. It's a time intensive process so we prefer to wait until we have a larger batch of lines to record.
It would be nice if we could instantly put in lore lines for new champions as they come out but it just isn't practical to bring in multiple actors to record a couple lines.