The Wikipedia article on the [Jubilee line](https://en.wikipedia.org/wiki/Jubilee_Line_Extension) doesn't mention hair specifically, but does mention they're to improve airflow, prevent jumping or failing people, prevent litter and dirt circulation. I always imagined it was in preparation for the move to automated lines much like the [Docklands Light Railway](https://en.wikipedia.org/wiki/Docklands_Light_Railway) which demonstrated feasibility in 1987 (11 years before Jubilee).