Making a Linux home server sleep on idle and wake on demand (2023)

I considered doing something like that but eventually I went for the simpler solution of plugging my little servers into smart plugs. I shut them down, then power off the plug over wifi. I start them by powering on the plugs. The plugs draw very little power. The servers are ARM SoC and draw 1 to 4 W. One of them has an HDD that draws about 10 W but I can unmount and power off it when I don't need that disk and I still need the server on (it's also got a SSD.)

Sounds like a neat setup - can you self host the smart plug control (Home Assistant?), or do they connect to third party servers?

It's a Shelly S. Among the many ways to control it I'm using its local web server to turn on and off the power from my phone, and a curl to do it from my scripts. I'm also polling the url for reading the power level and know when a shutdown is complete, before turning off the power. I think it can be interfaced by home assistant and similar platforms but I never tinkered with them. Self made small scripts are more than enough for my use cases.