Left in FTTC jumpers go to an allocated DSLAM port.
If all the other DSLAM ports are in use/allocated to customers working services, then there are no spare ports for any new services that might need connecting.
However, if there are lines running though a DSLAM port which is not used, ie the line is stopped, and/or the DSL service is no longer required, then Openreach visit the cab and make the original line a straight DEL , then use the newly freed up port to provide service to the new customer.
The above is usually done on the same job instructions, so the engineer’s notes might say ‘recover jumpers from line XXXXXXXXX , to reuse on line YYYYYYYYYY’
Hope this explanation helps.