This is not a sane way to install third-party software because by adding their repository, you allow them to "upgrade" and package in your system, including Linux kernel, bash, sshd. Is your system secure if companies like Microsoft, Sublime Text, Slack Inc have root access to it?

So you either give them root access or install and update software manually. Because package managers like apt do not have a sane method to install third-party proprietary software.

And even with open source software, it is a bad method. For example, a developer that provides a Debian repository with different versions of PHP, has overwriten openssl library for users of his repository despite it can break something else.

So there should be a safer way to install third-party software.