The "ongoing maintenance cost of software" can also be the additional complexity in the mental model of developers that have to deal with nearby parts of the codebase. Even if no maintenance is needed for a specific feature, it doesn't always make sense to keep around.
There's also knock-on effects, e.g. an offering like this could attract the type of user that would add a lot of additional support burden for the value that the company _and_ the user gets out of the feature.
This is a poor excuse. I understand that MSFT/Github feels compelled to provide a convincing reason to users, but this is not it. 99.9% of problems are caused by developers making changes or pushing new code. I've been writing software for close to two decades now--if you don't touch it, it's not going to break on its own.