Looking at a large chat history, it's not visually obvious, (except by reading the dates, which is a distraction from which is a distraction from content) that messages are old, or read previously.

Under Gossip, I usually used the Conversation:Clear option to clear things I read, so if i wanted to read them again, I'd go to the log. Messages in the window were always things I hadn't read yet.

Xchat uses a red marker line to distinguish between the last time you looked at the window from newer content.

Gajim has a nice feature where the color/text-size of old chats are dimmed/shrunk, so you can read them, but they're again clearly distinct from the new content.

Any of these stratagies would make empathy feel more comfortable to use.