This normally would go into the "Updaters" subforum, but those posts are from years ago, so presumably it's dead. Anyhow.
Using client v5, version 188.8.131.524. (Maybe that's out of date, but it's not updating itself either.) Starts up with no errors, contacts the server according to its logfile, doesn't report or log any errors or other problems. But... there are never any updated addons reported for several weeks now. Looking at each addon's specific page, there are definitely new versions, but the client's not seeing them.
Is v5 just completely dead now, in favor of Twitch?
Okay, I am certainly not going to try to twist anyone's arm or put the proverbial gun to your head, but....
There IS a standalone version of the app. You are not required to use the browser version.
Twitch App is compatible with both Mac and Windows, something the Curse Client hasn't been in a few versions.
As soon as Twitch bought Curse and introduced the Twitch App that was the sign to switch, because...
The Curse Client hasn't had updates in a very long time, yet Twitch App gets them regularly.
Also, I recall (but can't find) a post or message well over a year ago that explained that the Curse Client was obsolete and no longer supported, and it was replaced with the Twitch App. Regardless of me not being able to find that message, the mere existence of the Twitch App was a crystal clear indicator that users ought to switch.