I'm not sure why they sought to do this via a filter now instead of just a highly visible color coded tag like the [Solved!] tags.
We actually do have a Solved thread prefix, but it wasn't applied retroactively (nor was Question or Discussion). You can see one on this thread: https://forums.tomshardware.com/threads/quick-compatibility-question-surround-memory.3448425/
There's way too much space for stuff on the right of the threads.
You can collapse either or both sidebars. If you click on the hamburger menu next to the TH logo it will collapse the left sidebar, and the three dots in the top right under the Alerts link will collapse the right sidebar. You can collapse them by default via your profile preferences here: https://forums.tomshardware.com/account/preferences