I would just drop the icon into BTT as-is and set the "icon size" attribute to 21x21. Now I'll go do it and see if that works
EDIT: It indeed did exactly what I expected it to do. Screenshot:
Of course, your hypothetical icon is 21x21pt, which is within BTT's default 22x22pt bounds. So if you wanted to avoid editing the BTT attributes and keep your widget's icon size at the default 22x22pt, an alternative option would be to resize the canvas of the source image to 88x88px like so:
You get the exact same result using this in BTT with the default icon size (22x22pt):
I'm not really sure what the "issue" you're trying to point out is, but I think this exercise pretty clearly illustrates how BTT resizes all icons to 22x22pt by default. As long as your icon has been rendered with any integer scale factor of 22x22pt, it will appear @2x on the Touch Bar.
What I was trying to say earlier about Apple's assets is that they are already pre-rendered at the correct size, and in fact are the assets used by the system itself, so you should NOT use the PDF versions if you want the most pixel-accurate rendering possible. Instead, fit all icons 22x22pt and under within a 22x22pt bounding box. For icons over 22x22pt, fit to a square of the same size as the icon and then adjust the size in BTT manually to the correct larger size (like 24x24pt). There is no way around that manual editing step if you want an icon larger than 22x22pt.
I feel like this gives a pretty comprehensive overview of icon sizing actually, haha. So someone could use this info to prepare the assets in this thread. Most of them are 22x22pt and under, so can be used in default BTT buttons without any editing so long as they are set in a 22x22pt bounding box.
EDIT2: To be even more clear, any anti-aliasing artifacts you see are not caused by improper sizing, but by the way the Touch Bar itself renders images:
You can clearly see that the icon has some anti-aliasing, even though it is the correct 42x42px size it needs to be to render properly. Is that the issue you're talking about? If so, it seems like the native Touch Bar UI does the same thing. @Andreas_Hegenberg is there a story here?