Purely optimistic STM implementations that abort transactions early and don't permit other transactions to read uncommitted data can guarantee forward progress, and I believe that both Haskell's STM and Fraser and Harris's STM do, though I could easily be mistaken about that.
yes, 'synchronize' uses a try_lock/backoff algorithm, same as std::scoped_lock.
edit: it could theoretically livelock, but I believe most if not all STM implementations also do not guarantee forward progress.
Purely optimistic STM implementations that abort transactions early and don't permit other transactions to read uncommitted data can guarantee forward progress, and I believe that both Haskell's STM and Fraser and Harris's STM do, though I could easily be mistaken about that.