Hi, is it possible to stop Mailspring showing deleted messages from threads in the deleted items?
Not entirely, because Mailspring is conversation-based: a thread shows up in Trash as soon as any message in it is deleted, which is why the bar at the top says “threads with deleted messages” rather than “deleted messages”. The part you can rely on is that when you open a thread from the Trash, Mailspring only shows you the deleted messages in it — the ones still in your inbox are hidden — and the reverse is true everywhere else, so deleted messages don’t clutter the thread in your inbox.
If what you want is a per-message, non-threaded view, that isn’t something Mailspring can do today; there’s some background on why here: How do I disable threading / conversation view? — Mailspring Documentation. And if it’s just about keeping the Trash tidy, the Empty trash now button in that same bar wipes it permanently.