If you load the editor from editor.construct.net
it always auto-updates. There's a complicated mechanism using a Service Worker to handle that which I think would be difficult to change, and besides I think it makes sense that the base domain always serves the latest stable release (it's not like a new user visiting for the first time wants to load an old release there). If you install Construct as a desktop app, it does exactly the same thing as the browser, so if you install from editor.construct.net
that's still the update process you'll get.
If you load an old stable release in the browser it does still notify you. I actually thought we'd already added a setting to cover that, but I must have forgotten. I've added a setting to turn off update notifications for the next release, so you can make sure loading old releases doesn't keep prompting you to update. Note however this does not affect the update process for editor.construct.net
: as the actual files served there are updated, then it will still update you even if you turned off update notifications, and so the editor still also shows an update notification to tell you that's going to happen.