All major announcements will be in the Announcement category of these forums (I highly recommended turning on notifications for new threads being created in that category). and those announcements will also be on the TwitchDev Discord server.
As well as those announcements, for in this case the shutdown of v5 was also announced as deprecated on multiple TwitchDev streams, deprecation banners have been all over the v5 documentation since it was announced a couple years ago, and additional banners to include links to migration guides and the shutdown timelines when they were announced.
As well as all of that, make sure you also have emails enabled in your Twitch settings as multiple warnings have been sent for months now for any apps that had been using deprecated endpoints, and in the future Twitch will likely continue to send such emails when if another service was to shutdown in the future.