Still in the "smart" era, but the Motorola Atrix allowed that, but with its own laptop form factor dock.
https://www.cnet.com/culture/how-does-the-motorola-atrix-4g-...
I am pretty sure that modern phones are more than powerful enough! My wife's iPhone 16 Pro Max would be amazingly useful if not limited by iOS (which always feels like it's hiding true capabilities behind an Etch-A-Sketch interface to me). If you could plug the iPhone in and run a macOS desktop (which hasn't really changed for 15+ years), that'd be great. Thanks in advance.
I have a POCO F7 Ultra which is powerful enough to run LLMs via PocketPal and could easily replace my daily laptop or PC for work if it wasn't scuppered by USB2 on the USB-C port. If I could easily run ollama on the phone via a web interface I would because it's faster than my main PC for LLMs I think!
On Android you can go into Developer settings and force enable the ability to use desktop mode but sadly I can't without proper display support on the USB C.