Don't think AdGuard for Mac, Windows or Android will ever be made open-source, it doesn't make sense to do that at all.
The AdGuard Adblocker browser extensions, AdGuard Home, AdGuard for Safari, AdGuard for iOS, AdGuard DNS, etc. are all open-source though.
And it's worth pointing out that requiring an app be open-source shouldn't equate to some sort of piece of mind. If anything, a potential hacker could view the source code and find vulnerabilities they likely couldn't find (as easily) if it wasn't open-source and take advantage of that.
There's nothing wrong with proprietary/commercial software, regardless what the FOSS community says. And there's nothing wrong with keeping proprietary/commercial software closed source either.