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In this day and age, it's easy to verify whether or not that's the case. If it is, schedules nowadays are so tight that the original delay causing the knock-on effect could be several sectors ago. I can't say, as a delayed pax, that I would be particularly interested in knowing what caused it.
When you say in these days - ATC and missed slot delays have been easy to identify since just about ever at airport information desks !
Telex based service has been around and used as long as schedule flights have existed !
The fact is many travellers are unable to comprehend that their getting to the gate 2 minutes late after their personal shopping on that 6.00 am departure to Malaga and consequential missed slot would lead to hundreds of other passengers being inconvenienced throughout the rest of the day !