It's not a career killer, it's a specialization. Focusing on nodeJS JS/TS frontend+backend stacks will equally disqualify you from .NET/Spring shops (against an equally qualified applicant, at least). The same goes for making the switch to gamedev or systems dev from many years webdev (you can do it, but you'll probably be dropping back down to a Jr or Mid-level IC position), going from COBOL/Erlang/Rust to another stack, doing Salesforce (or other CRM/specialty software) development, etc.

It's going to happen to anyone after 10-15years in the industry.