Just my 2ct on this...
I like the idea very much

I think it could help to improve the update entries.
https://travelmapping.net/devel/updates.php features the 'File Root' column. A new (automatically added by frontend) column could open showroute with dbname=TMxxxxxx where xxxxxx is the last stored alternate database before the date indicated in the 'Date' column. That means, with highway data and user data from the day (or week, or month or year) before the update was made.
If a routing was heavily changed, I usually check my travels on mapview. It would be great if I could directly switch to
mapview with the current view.
I also love the idea of seeing stats and mapview at a previous point like before my last travels or at the end of last year

To be honest, dbname is not yet useful for me since it's not persistent. We could simply add a yellow banner - like the red 'update in progress' banner - that indicates 'You currently view historical data from xxxxxx'.
Having this in mind, I think that we should minimum have one historical database per year. Likely from Dec 31. A simple drop down menu could make the trick to select it on the front end. Could we also do this for 2015 to 2024 data? It's all on Github, but I assume it's hard to automate, isn't it?
If data space is not a big issue, I'd prefer having one historical database per month to better meet the use-case demands from above. Then, we would likely need a more sophisticated selection option.
I don't know what could be broken and how to test things on tmstage now. I cannot simply click around since the QS is not persistent. But all I quickly tried worked fine. Thanks for your work
