Advanced search is currently your best option.
I'm curious what kinds of sort+filtering you'd like to see in an easy to reach way?
It sounds like:
- Filter: Unsolved, Sort: Latest Topic
- Anything else?
We are adding a plugin with additional sorting options (sort by more recent topic, most recent post), with the idea of then adding to the list popular ways folks like to sort and filter the forum. (Its development is way slower than I had thought it would be. As we added it to the forum, either it broke or it seemed to break the forum, sorry).
I also know folks have asked for a way to see a subset of categories, tags, and other filters, and subscribe to that list of topics. (E.g. Show me all unsolved threads in any of the Categories: shiny, shinyapps.io, R Markdown, Or with Tags: rmarkdown and shiny). A form of this is popular on reddit and stackoverflow.
This currently isn't possible on discourse but is definitely something I'd like to see and will get around to supporting plugin development on.