Thank you. It is helpful to know I was not missing anything. Basically, Shopify does not encourage keeping themes up to date and thus the /shrug.
- no notifications
- no way to check without downloading the theme
- no change log
Normally I would schedule a "maintenance cycle" when I actually know that something needs to be maintained but sounds like a randomly selected time interval might be the best way to go.
Also some third party themes keep a version in the theme settings but again you have to go out and reference|download
For the most part critical frontend bugs are just rare, or done by user error so fixes and minor releases aren't that newsworthy in regard to the merchant POV.
However major updates follow platform level feature releases, like theme sections. So that's generally referenced by year 2016 because there is also no real version numbers on themes either. Which can also be considerd a consequence of shopify core themes using the same framework|buildtool like slate( at one point the timber framework) and that changes quite often so propagating that onto individual theme versions would create a constant deluge on merchants with daily/monthly version number changes.
Things this year to keep an eye on are COGS and multi-currency until the next unite
Longterm keep an eye on changelog.shopify.com, github.com/shopify/slate, the liquid help docs or the cheatsheet and partially the github liqud repo( that is the default open source liquid template engine, it doesn't have all of shopifys specific syntax baked in but some features start there.)