That simply isn't true. Things that are funded by an appropriations bill are considered discretionary because the level of funding they receive is up to the discretion of Congress. Things that are funded by a mandate are not considered discretionary because their funding is not up to the discretion of congress from year to year. The opinions of liberals or conservatives do not factor into which is considered which. There is a precise legal meaning here. It is not simply "a term the left made up".
In addition to there being a legal distinction, there is also an accounting distinction. Medicare and social security, the two biggest non-discretionary expenditures by the federal government, come out of entirely different piles of money than discretionary expenditures do. Were you not aware of that?
Your income taxes are not funding social security. Your social security taxes are not funding things that come out of the general budget. Cutting social security benefits would not free up more money to fund the things you think government should be doing.
So, although I get that you do not like the nomenclature, I don't get how it makes some particular thought unthinkable. They are two different things. They should have two different names. It seems far more Orwellian to pretend like there isn't a legal difference between the two, simply because it would make achieving political goals you happen to find favorable easier.
You seem to be under the impression we could take the money from social security or medicare and spend it on stuff you like better. Legally, that isn't true. So yes, in the sense that it would be totally freaking illegal, it is "unthinkable" to cut social security in order to lower income taxes.
You might as well refer to calling animals "lions" and "tigers" as newspeak, because they make the thought that they are actually the same species of animal unthinkable. Well, they aren't the same species of animal. So it ain't newspeak, it's just nomenclature that you don't like.
It is perfectly accurate to describe spending appropriated from the general fund as discretionary and social security as non-discretionary whether or not you like the terminology. They're two different things, one of which is up to the discretion of congress, one of which is not.
Come up with a new term if you'd like. But you've strongly implied you think the two types of spending aren't different. That's not Orwellian -- that's you simply not understanding freshman high school civics. You also claimed that "discretionary spending" is a term the left made up. Are you going to post a citation for that, or admit you were wrong?