Diablo IV might be right around the corner, but that doesn't mean Diablo III isn't getting new stuff to do. Diablo III PTR 2.7.5 (Public Test Release) is inbound and Season 28 will soon follow.
What are seasons and is the content from those seasons which have already passed still available? If I start playing Diablo 3 eternal edition today would I for example get all the content?
Seasonal content changes every so often, and once one season ends (season 27 is on the way out now and probably only has a week left) you'll have to create a new character and start over again with the next season. None of the previous seasonal content is available. Season 28 has an entirely new aspect of unlocks to earn, and even though I have played almost every season through season 27, 28 is really adding some pretty cool stuff to work towards.
You could start a season 27 character just to get the hang of the game, but I wouldn't gett to involved with it until S28 drops probably in 2 weeks.
Season 28 sounds like it will take a while to complete.