Fairly simple, when i call up the tab overview (Shift-Cmd-\ ), pinned tabs show an 'x' and can be closed like any other. I accidentally closed several pinned tabs I wanted to keep in an effort to do some general cleanup. I am scrambling to try and recreate them, remember which tabs were there and re-load and re-pin them.
see above
Version 0.99.130.3.1-rc (WebKit 621.1.2.111.4)
Sequoia (15)