For EASA OPS requirements, must an airline transport airplane be fitted with an autopilot system?
Indeed EASA requires "autopilot" but they don't state it directly like "every airliner flying in the European airspace must be autopilot equipped". They require it indirectly.
When I first read your post I immediately thought of RVSM. I recall talking to an ATCO (retired now) about automation on board, and he told me that without autopilot you cannot fly in RVSM airspace.
So I had a look in skybrary concerning RVSM and they did the dirty job for us. They mention all 3 EASA regulations
IR-OPS SPA.RVSM.100 and SPA.RVSM.110,
EU-OPS 1.872 and they have quoted the interesting parts:
An operator shall ensure that aeroplanes operated in RVSM airspace are equipped with:[...]
An automatic altitude control system
Emphasis is mine.
There might be other regulations requiring other automatic flight components but I am not aware of them although a quick look in the documents reveals references to auto-land systems as well so these documents cover more than RVSM.
The summary is that EASA does not simply specify "you need autopilot if you fly an airliner" but they go into detail, of what they require and for what reason.
RVSM Reduced Vertical Separation Minima
ATCO Air Traffic Control Officer