Looks scary, but it's really not
Existing software on your system loaded a system extension signed by "Adguard Software Limited", which will be incompatible with a future version of macOS. Contact the developer for support.
If you're using AdGuard for Mac, chances are you encountered this message. What's all of this about? Let us explain.
As you may know, AdGuard filters all network traffic on your Mac, it's necessary to be able to block ads and tracking. And to have access to this traffic, AdGuard installs a so-called kernel extension, which is referred to as "system extension" in the warning message. This kernel extension is based on Apple's "Network Kernel Extension" API.
Now the problem is, this API will be no longer supported in the next version of macOS (10.16) and will be replaced with a new "NetworkExtension" API. Don't let the similarity in names confuse you: these are two very different APIs, and AdGuard needs to be updated in order to work with the new one.
However, this is not a disaster by any means: we're already working on it and we'll almost assuredly have finished the job by the time of macOS 10.16 release. In fact, judging from Apple's statement to developers, it looks like you'll be able to allow apps with old APIs run on 10.16 anyway, it just won't happen by default.