SmartLocks by default keeps a History of about 7 days. You need to scroll down to the bottom of the page and click on Load More to see the history. This number can’t be changed and is controlled by SmartThings. I believe at one point it used to be 30 days but of late it’s about 7 days.
There are other ways to access logs, specifically you can get your device event from the device log history in the IDE. That’s more raw information and not filtered the way you see it in SmartLocks. You can also check out Simple Event Viewer logging SmartApp if you need to log your events for archival/other purposes to google sheets.
BTW, the Lock User Management (LUM) app works with the new and the classic ST app and you can use in conjunction with SmartLocks (the history logging tab is unique to SmartLocks and LUM logs the information and usernames with SmartLocks so you can have the benefits of enhanced user management with the logging benefits of SmartLocks).
On the flip side apps like LUM log events to the notifications tab (unlike the history tab) and that information is kept for a very very long time (I can’t say forever since it depends on ST but ST’s business model depends on analytics on this usage data from users so they keep it for a long time) and you can access it through the mobile app or through the IDE. EDIT: Looks like this has been limited to 5-7 days as well now.
SmartLocks was designed to fill a basic gap in the user experience it wasn’t designed for power users or for commercial/heavy use purposes. For that matter the Basic version of LUM (which is free on the same topic) logs to the notifications tabs so you get the ability to program users and one additional feature over SmartLocks but there you miss the History tab (ST in it’s infinite wisdom decided not to let devs access more classy looking UI components). So it’s a trade off between a dedicated history tab and basic access or customized apps with advanced features and having to access the notifications tab for (virtually) endless logging without filters.