Graph prototype documentation includes clear description of parameter "discover":
https://www.zabbix.com/documentation/5.0/manual/api/reference/graphprototype/object

discover integer Graph prototype discovery status.

Possible values:
0 - (default) new graphs will be discovered;
1 - new graphs will not be discovered and existing graphs will be marked as lost.

The parameter is also returned when graph.get method is used, but it's not documented for the graph object:
https://www.zabbix.com/documentation/5.0/manual/api/reference/graph/object
The same seems to be true with items and item prototypes and triggers and hosts.
This can confuse users, all parameters coming from api should be clearly documented, even if it's just stated that this parameter is not used for this object.