https://dgross.ca/blog/linux-home-server-auto-sleep did the rounds lately.

But you'll need another system to always be on to handle this.

In many cases, you can "fake" this in other means. For example, I had Remmina configured to run a script to send a WOL packet and wait before connecting via remote desktop to a computer.

Well, you can in theory enable WOL without Magic Packet, but that means any network activity will wake it up, even a random ping, so you might find your PC waking up randomly.

If your computer boots when plugged in (not only by pushing the power button) it would be feasible via a connected plug, but you'd still need a small computer, e.g HomeAssistant on a RaspberryPi, to convert the HTTP request to ZigBee, or a WiFi plug but I'm not sure which one would handle an HTTP request, if any.

Some motherboards explicitly enable wake on LAN as a BIOS option. If not in the BIOS it's going to be a bit harder, but the software option recommended, (the Archlinux forum link) looks interesting.

You can accomplish this using your router. The router can send the magic package to the computer. You can do this remotely from over the internet and even through a VPN connection to your router (preferred). If your router doesnt support the functionality out-of-the-box, OpenWRT does!