Not really Steam's fault, their app is built in a chromium browser, which stopped supporting those OSes a few years ago. A perfect example of Google having too much control over the Internet. This is what happens when big companies are allowed to purchase their competitors.
Edit: people in this thread are either really forgetting how much trust google used to have with basically the entire Internet. They were seen as the "good guys" for a long time.
Or they're forgetting how unique and revolutionary chromium based desktop apps were when they first came out. It is a colossal pain in the ass to create a modern browser, if you have a web page in your desktop app like steam does, it quickly became a very difficult, time consuming, and virtually fruitless endeavor to develop a headless browser just to sit within your desktop app when you could just go with chromium.