Since Congress passed The Constitution's Appropriations Clause and the Congressional Budget and Impoundment Control Act (ICA) in 1974.

Because otherwise, the executive could just unilaterally shut down any part of the government at any time. Or siphon money from one department to another.

Which it can't by design, since congress controls the purse.

There's a process outlined in that act, under which the executive can report to Congress that it is reducing spending, and Congress must approve that reduction in order for it to take effect. That is the law of the land. The law is being broken.

If you don't like the budget, there is one governing body that can do something about it in this country. The legislature. They control spending, just like putting people in prison is controlled by the judiciary.

Strangely, all the people grousing about executive overreaches are dead silent on all this.

The sin has never been executive overreach, the sin was always an executive they did not control.