My first Linux install was Slackware sometime in the late 90’s. I didn’t really use it though, as I never managed to get it working with my dial-up Internet. Stupid winmodems.
The first distribution I actually used was Mandrake. Others I’ve used since then include Suse, Ubuntu, Debian, Mint, Manjaro, and EndeavourOS. I’ve landed on using Manjaro on both my main desktop and laptop, though I have secondary machines running Debian, Slackware, Ubuntu, and EndeavourOS.
The ancient systems I knew didn’t have a power button, and instead power was controlled by a physical switch on the high voltage side of the power supply.
The key actually locked out the keyboard, which was possible since the keyboard had a dedicated connector. So you could still turn the computer on, but you really couldn’t use it.
I suppose locking out the power button is a suitable replacement for a modern case.