My idea is to have a small dot next to the game's workshop button when a mod is uploaded or changed. It will have a bit of higher overhead as you have to keep/store a user's state in a text file to know when they last checked the workshop page. Based on the sort 'Most recent,' consider also showing the dot in one color for new mods and another for updated mods.
Additionally, the current system supports the following authors, which can also be helpful.
I can understand if this isn't a support matter and or isn't an idea Steam as a service is thinking of doing...as it's got Ui and backend changes. And yes only seems to work when the client is online vs offline.