That's ironic. At least in the brief time I used Windows, Trillian had the worst dumpster fire of a UI. I can't say I really liked Windows UI, but they didn't even attempt to conform to it to make it half-usable.
But that might be related to me hating the UI of WA, Signal, Telegram etc. Even Gajim switched to this shitty UI style (sidenote: Any usable Windows XMPP/Jabber client that still looks good, with good = "userlist, plus chats in separate windows"?), so I’m starting to think it might be possible that most people don’t hate it ;)
edit: There’s AstraChat which looks nice and apparently supports WA, alas, it’s commercial, and they don’t seem interested in selling to non-enterprise users.