Due to the need for perfect synchronization all users need to be using the exact same version. Mods can also break between versions. It is therefore very common for public servers to stick on one version for extended periods of time. It is common for people to use the Steam "betas" functionality to pick an exact version or download an exact version from the Factorio website.

I would say that servers only tend to update when large features are released. So announcing a security vulnerability would likely push some servers to update.

Without metrics of some kind from Wube I guess we aren't likely to know for sure, but I doubt very much it is common to run old versions of the game on Steam. I bet you that most people are simply running on the latest version at all times. That solves the MP issue, and plenty of mods don't need to be updated for each game version.

Factorio is special though, because it actively uses the beta version functionality in Steam to not only provide betas but also older stable versions. This allows the devs to move fast and break things.

I know I've held back my copy of Factorio due to some concern over changes in newer versions, preferring to letting the dust settle before upgrading to the latest stable version.