YOE 7+ Tc 12lpa Heard tech stack is very old.
Depends on the team. Some teams work heavily on old code base. Some teams have a mix of old and fairly new codebase.
How bad it will be if one gets to work on old codebase future career wise?
It’s not realistic expectation to work on fresh or new codebase always. Old codebases help to learn what-not-to-do, which is important to build expertise in career. Moderate doses of it would be your best bet.
No one knows?