C2 Plugins and Behaviors List
Latest version, I guess you have to keep an eye on the official topic of the addon you want to update.
Subscribing to the topics themselves may be a good idea as well I guess. Link is at the bottom of the page for each topics. (Subscribe topic / Bookmark topic)
Otherwise, for new addons, I've been keeping a list of monthly novelties for the past three years.
Every beginining of the month they are listed (amongst other things) in the sticky post in the Construct 2 General forum : What you may have missed...
And, to keep track during the month, you can do as I do, that is to say checking the index of the forum and seeing the date of creation of the topic.
If it is in the current month/year, pretty safe bet it is a new addon.