I used a slide rule for some of my high school physics tests in the 2010s, just for fun. I lost a few points from messing up order of magnitude calculations, but it was totally worth it. I've since collected a few more slide rules from estate sales in my neighborhood. I wish more people knew about them—they're such a neat and elegant part of engineering history, which is why I keep them around.
I have no use for them on a day-to-day basis, though. An abacus is more useful for things like counting board game points and adding up taxes.
2. I use QWERTY on mobile devices, Dvorak on standard keyboards, and RSTHD on split ergo keyboards, and I can switch between all of them without any hassle. It helps if you use a different keyboard form factor for each layout, but even if not, I don't think it's difficult to maintain adequate fluency on QWERTY when you need to use someone else's computer.
I'm proficient in Qwerty, Dvorak, and RSTHD [1]. I also learned Colemak at one point and was proficient at it, but learning RSTHD replaced my Colemak muscle memory (which in itself is odd and interesting). Right now I use RSTHD when I have my ErgoDox, Dvorak on a standard keyboard, and Qwerty on mobile. Some additional things I've found:
* It's possible to be proficient in multiple layouts at a time as long as you use all of them on a regular basis.
* Furthermore, it's possible to separate your muscle memory for a layout from your muscle memory for the physical keyboard enough to be able to switch between keyboard layouts basically at will.
* Getting used to moving around punctuation is much harder than getting used to moving around normal letters for some reason.
* If you use different keyboard form factors (for me, ErgoDox vs. standard keyboard), your muscle memory for the layout is tied to your muscle memory of the physical keyboard itself. I can't type Qwerty on an ErgoDox but I can just fine on a standard keyboard.
<aside>It looks right to me. The adjectival clause describing the OS says "the Chinese government wants the OS to replace Windows". There's an implied "that" between "the OS" and "the Chinese government".</aside>
I have no use for them on a day-to-day basis, though. An abacus is more useful for things like counting board game points and adding up taxes.