Depends what you mean? Mathematica is quite impressive.
Many symbolic options exist in lisps. https://stackoverflow.com/questions/10355112/why-is-lisp-so-... is a good answer that goes over some of the reason for that.
Mathematica is certainly as powerful as a TI-89, but not OSS.
Right, though you can get a free license with raspberry pi, I think? https://www.wolfram.com/raspberry-pi/
I'm sure there are restrictions, but likely not a concern for hobby interests?
As an adult and OSS enthusiast, I've often wondered if there is an OSS option that can at least match, and ideally exceed, the TI-89's capabilities. Is SymPy it? I've had a few reasonably good experiences with SymPy, but I don't know much about the theoretical underpinnings of CAS, or how SymPy compares to competing offerings.