(I wrote a long reply to this post with technical details; then I accidentally reloaded the tab in Chrome and it lost my textarea text and it's 3am here so argh).
But to briefly summarize when I did write:
> installing PS/2 ports on modern motherboards
I haven't seen a PS/2 port on any brand-new (non-industrial) computer, even rackmount servers, since 2009. My own last mobo with PS/2 ports was bought in 2008; I haven't seen PS/2 on a laptop since 2003.
> and just contributes to e-waste
e-waste is a problem, yes; but PS/2 ports are really, really, not a meaningful cause of any problems in this area.
> I also wonder how much power those "super I/O" chips are wasting with this unused circuitry.
Zero. ICs (mostly) consume power only when their transistors undergo a state-change. If the PS/2 microcircuitry in an IC is never used, then there won't be any concordant transitor state-changes, ergo, there won't be any (measureable) power-draw. This is why microprocessor lithography is great: you really do get stuff for free.
> why is PS/2 internally used for touchpads anyway?
I wrote "ostensibly"; in reality, it usually isn't actually "PS/2" as-we-know-it, but is some other protocol over I2C or even SPI as you suggested (e.g. Synaptics calls theirs "InterTouch"[1]), but the hardware-interface that the OS uses is compatible with Intel's i8042 which was the original PS/2 ( https://wiki.osdev.org/%228042%22_PS/2_Controller ) even though the i8042 no-longer exists today.
[1] https://investor.synaptics.com/news-releases/news-release-de...
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As for why practically every laptop seems to use "PS/2" for their touchpads today, I speculate that laptop OEMs prefer this "PS/2-a-like" for 2 reasons:
1. Consumes less power (and with far less overhead overall) compared to an internal USB connection, which is important in a laptop computer.
2. Laptops likely only have a single USB root controller, which is usually for the latest USB revision when the laptop was engineered - so a new laptop today likely has only a USB4 controller, which will be backwards-compatible with older USB devices, but you'll have issues if you need to use an OS or EFI application (e.g. BIOS Setup utility, Bitlocker manual unlock screen, custom boot-manager, etc) due to spotty support for future USB controllers, so it makes sense to ensure that the laptop's fundamentally-essential input devices (i.e. mouse and keyboard) are connected via a mature, widely-supported interface like the i8042's) to avoid the laptop becoming completely unusable if something goes wrong with USB support (because it's not like you'll be able to plug-in an external USB mouse/keyboard and expect that to work...). Whereas on desktops I've noticed that whenever there's support for some cutting-edge USB revision (e.g. USB 3.x in 2012ish or USB4 today) the board will have a couple of USB 2.0 ports via a different controller, so you won't be SOL if anything goes wrong with the new stuff.