PostgreSQL 11 error: column p.proisagg does not exist
In Postgres 11 proisagg was replaced with prokind in the system catalog pg_proc:


  prokind | char | f for a normal function, p for a procedure, a for an aggregate function, or w for a window function


The query needs to be adapted. Like:

SELECT ...
FROM pg_catalog.pg_proc p
 ...
WHERE p.prokind = 'f'   -- to only get plain functions
...


Related:


How to drop all of my functions in PostgreSQL?
PostgreSQL: How to list all stored functions that access specific table


The error you observed from the \df meta-command in psql is most likely due to using an outdated version of psql. psql 11 and up are updated to deal with  this change, of course.