It has been nearly two years since the launch of YouTube Red, or as I like to say: The subscription that lets you actually watch YouTube. Prior to that, it was called YouTube Music Key and only served music videos and enabled background and offline playback. Now it looks like there might be a new subscription tier called YouTube Plus coming soon.
The features discussed below are probably not live yet, or may only be live for a small percentage of users. Unless stated otherwise, don't expect to see these features if you install the latest version.
A teardown of the YouTube v12.37 apk turned up two new lines of text representing subscription options. Naturally, one line represents YouTube Red, but it's the one that names YouTube Plus that is of interest. There are currently no references to Plus or any other subscription tiers in YouTube's online documentation.
<string name="unlimited_brand_name_red">YouTube Red</string>
On a possibly related note, a Chrome extension named YouTube Plus was pulled from the Webstore back in April. It was later reinstated after the developer changed the name to Particle for YouTube. An explanation was later given that the extension's name violated the branding guidelines of the Webstore – the Google Play Store has a similar policy regarding product names. It's possible this was a coincidence, but my suspicion is that YouTube became aware of the conflict after coming up with the Plus name, then reported it to the Webstore at that time.
Unfortunately, details about Plus are pretty scarce. But there is one clue about the feature list for Plus: Background playback will probably be included. This much appears likely based on the line below, which is shown to users in a notification when they have a video playing and switch away to another app. It was updated in the latest version to replace the YouTube Red name with a placeholder (highlighted). This was done so the app will show whichever subscription name is appropriate to the user.
<string name="background_settings_notification_message">With YouTube Red, videos keep playing with the screen off. You can change this in settings.</string>
<string name="background_settings_notification_message">With %1$s, videos keep playing with the screen off. You can change this in settings.</string>
Subscribers to YouTube Red also enjoy ad-free videos, offline playback, and exclusive access to YouTube Red original programming. They also have full access to Google Play Music. These features aren't mentioned in any relation to Plus, nor were there any other lines of text updated with placeholders, but that doesn't mean they aren't included with Plus. In fact, none of the marketing text has been updated. In other words, it's not even clear which subscription is superior. It's possible Plus is a subset of Red with just a couple necessary features, or it might build upon everything available with Red and add something else. For example, Plus might be an all-inclusive subscription package with both YouTube Red and YouTube TV.
Pricing and availability are also a mystery. At $9.99 USD per month, Red is generally considered a reasonable price for most people that watch YouTube regularly enough, but there are other markets around the world where that price would be far too high. In those areas, a less costly alternative with fewer features might stand a better chance. Such a plan might not even be available to locales that have access to Red.
For the sake of being thorough, it's worth mentioning that Plus may simply be a scramble of the branding that will come with the merger between YouTube and Google Play Music. I feel like that's unlikely given the signs mentioned above, but it can't be ruled out.
Frankly, there are far too many possibilities to make any firm assertions yet. There may be more details leaking out later, but we may have to wait for YouTube to make an official announcement. We can't rule out Google's October 4th event for details about Plus, but I think it's far more likely that we'll get a separate announcement in late October, a bit nearer to the 2-year anniversary of YouTube Red.
The APK is signed by Google and upgrades your existing app. The cryptographic signature guarantees that the file is safe to install and was not tampered with in any way. Rather than wait for Google to push this download to your devices, which can take days, download and install it just like any other APK.