IIRC, they do forbid third-party clients from their network. You can build it from source, but you won’t be able to connect to production Signal servers.
Third-party clients would not necessarily be a bad thing. Signal has limited resources, and as such has to cut corners. I for one would love a native desktop client that’s not Electron bloatware.
The economics of consoles made more sense when computer power was expensive, and the choice was an underpowered home computer with so-so graphics and sound or a dedicated game machine optimised for drawing sprites and scrolling the screen responsively, with the extra costs subsidised by the price of (uncopyable) software. When PCs caught up, the consoles started looking internally like x86 PCs with souped-up GPUs (and, of course, draconian amounts of DRM baked in). Now with devices like the Steam Deck (and similar form-factor devices running Windows in game-console mode), there’s no real reason to buy a dedicated game-playing machine.