That's true that the "Do Not Log" button has gone away because it's been replaced by inaction. Basically, if you do not set it to "auto-create call logs" then it will only create a call log in Salesforce if you specifically press Save Log; otherwise it does not make a call log (it will still show the previous call in the integration area though in case you decide after the call is over that you want to save a log of it).
If I still have a question about this - in the absence of a Do Not Log button we have two options: a) capture all calls, even personal and non-business calls or b) run the risk of reps not logging every call and therefore our Salesforce activity not being accurate. It would seem to make sense that you would want to have users check the auto-log calls option but then on a case-by-case basis not log certain calls. Is this going to be re-addressed?