Perhaps even the alt-keypad method isn't universal in Windows apps. I'm not an expert on this, but I know of one example where it doesn't seem to hold true, at least for the 4-digit alt-keypad characters:

In an old (2008±) version of Bricscad (version 9), I can paste in the plus-or-minus character from Atlantis word processor (or from this message form), and I can create the character by using Bricscad's own code %%p or the Unicode \U+00B1, but I can't create that character by pressing alt-0177. In fact, it appears that I can't use alt-keypad to create any of the 4-digit alt-keypad characters in Bricscad, though I can paste them all in and create some of them by other means.

I can create at least some of the 1- 2- and 3-digit alt-keypad characters, for example the degree character (º) alt-167, a dot (•) alt-7, and the 1/2 character (½) alt-171.