Dependency tracking for UDFs is working. You can't drop an UDF until the last object that depends on it has been dropped. However, the message is incorrect. Full example follows:

SQL> create database 'depends4.fdb';
SQL> set term ^;
SQL> declare external function getExactTimestamp
CON> timestamp
CON> returns parameter 1
CON> entry_point 'getExactTimestamp' module_name 'fbudf'^
SQL> create table t(a int)^
SQL> create trigger tad for t after delete as declare dummy timestamp; begin dummy = getexacttimestamp(); end^
SQL> create view vudf(t) as select getexacttimestamp() from rdb$database^
SQL> create table tudf(a int, c computed by(getexacttimestamp()))^
SQL> create domain dud int check(value between extract(week from getexacttimestamp()) and 25)^
SQL> drop external function getexacttimestamp^
Statement failed, SQLCODE = -607
unsuccessful metadata update
-cannot delete
-UDF GETEXACTTIMESTAMP
-there are 1 dependencies

But the engine really tracked everything, as shown here:
SQL> show depend getexacttimestamp^
TAD:Trigger, RDB$2:Computed column, RDB$4:Computed column, DUD:Validation
        [GETEXACTTIMESTAMP:User defined function]
+++