It's common enough most mid-range and better motherboards (and many SBCs and cheap Intel/AMD/ARM desktops) include it by default. The chips for 2.5G NICs are about the same price as 1G parts now, and driver support is excellent across Windows, Linux and macOS.
It's basically a free 2.5x faster networking upgrade with the same cabling, so many are adopting it. And since it's backwards compatible with 1 Gbps networks, it's not a big issue to include it.
2.5G switches are typically a little more than 1G switches, but the prices have come down substantially in the past few years, making it a worthwhile upgrade instead of going to 10G, especially if you don't need all that speed (and heat) and the hassle of cabling/transceiver issues that inevitably crop up.
It's basically a free 2.5x faster networking upgrade with the same cabling, so many are adopting it. And since it's backwards compatible with 1 Gbps networks, it's not a big issue to include it.
2.5G switches are typically a little more than 1G switches, but the prices have come down substantially in the past few years, making it a worthwhile upgrade instead of going to 10G, especially if you don't need all that speed (and heat) and the hassle of cabling/transceiver issues that inevitably crop up.