that parameter is modifiable but it is modifiable in DEFERRED mode.. So you don't need to restart the database for making the new value affect the new sessions.. That means , only the sessions will affected by the changed value..
If you don't want the old sessions to work with the old value, then you need to restart the database.
This is already documented in Database reference ->
DEFERRED
The DEFERRED keyword sets or modifies the value of the parameter for future sessions that connect to the database. Current sessions retain the old value.
You must specify DEFERRED if the value of the ISSYS_MODIFIABLE column of V$PARAMETER for this parameter is DEFERRED. If the value of that column is IMMEDIATE, then the DEFERRED keyword in this clause is optional. If the value of that column is FALSE, then you cannot specify DEFERRED in this ALTER SYSTEM statement.