You can opt-out of that change, by using AppContext.SetSwitch("Npgsql.EnableStoredProcedureCompatMode", true);
(at the top of the page).
AppContext is a class that is meant for things like this, it lets developers define switches which are then read by referenced assemblies to switch on/off functionality. In the past they've defined more of these switches which should be fine.
I'm not happy with this change either, but this switch should take care of it. I honestly don't know why they introduce these changes as mandatory. Stored procedures (so not functions) in PostgreSQL are very limited in nature and this change makes it cumbersome for us too; as in: we have 20 years of usage with functions as procedures for postgresql and now we have to ditch that and introduce a new type: functions, and use the stored procedure type for the 'real stored procedures'. The internal nightmare around this brought us to the conclusion not to support 'real stored procedures' in the next version to begin with (new projects will be fine, but the big amount of existing projects aren't).
The one thing we're thinking about for the next version is to generate out the select so people can forget about that switch, but as it requires a special pipeline only for postgresql and functions, we've still doubts, as the switch mentioned above solves it too.