Old days
Gas engines: If you idled them for long periods of time they would "Gunk UP" Plugs and valves woudl get fouled and the thing just would not run right.
Disels however could idle all day
Today.. Diesels now do not care for ideling, They do not reach the proper operating temp and it's very hard on them.
Gasoline engines are now more forgiving, but again.. I'd not care to idle them.
Both old and new. YES the first short period of time is hardest on the engine, till it comes up to operating temperture. of aroudn 200 for Gassers not sure on Diesels.
Combustion produces many byproducts, some are acidic and can eat up your engine, When the engine gets up to temp these acids evaporate and do no harm.
Let it idle and they build up big time.