Self hosted means you host it yourself. I.e. you run the software. Running on your own home machine is definitely self-hosted. Hosting it on a VPS in some datacenter still is. Running it in a cloud platform somewhere is stretching it but I'd say that sill is. SaaS definitely is not self hosting even if the software is open source. The important part is that you're the one in control of the software.

If you run Plane on your hardware, then it is self-hosted.