There was an article in the Washington Post - probably you can't see this without a subscription, sorry - about the disappearance of encores. This was among pop music groups, most of which appear to be hard rock or punk groups, who say that they dislike the ritual or performative aspects of pretending they're done and then coming back, or that the process wastes time in which they could be playing another song or just leaving and letting everyone get home sooner.
The only pop music I attend is folk concerts at the Freight, and there the encore is always expected and entirely performative. You announce that what's next is your last song, there's a big final applause after it and you walk offstage during this, leaving your instruments, and then you come right back out and play another one. Then you're done. It's ritual, it's really rather silly, but the going off and coming back doesn't waste much time. I don't see that changing.
In classical, the encore occupies a different position. In non-pops concerts the final work is usually a big and heavy one, so the encore occupies the position of a light dessert, quite different from a pop concert where it's usually another song like all the others. (Though sometimes pop bands do something different: Steeleye Span used to do encores of acappella pop-song numbers after their concerts of electric folk.) Nor are encores entirely obligatory in classical but seem to be affected by the reception: I've rarely experienced an encore when the applause wasn't highly enthusiastic, and a performer doing a series of concerts on successive evenings may play an encore one night but not the next.
There are also unwritten rules in classical as to who performs an encore. Soloists in concertos with orchestra: sometimes. Whether the evening's schedule can afford the added time seems to be a factor here. Visiting soloists giving recitals: often. Visiting ensembles, from chamber groups to orchestras: usually. Ensembles playing in their home auditoriums: almost never.
Encores are traditionally rousing and lively, but in recent decades I've noticed a tendency towards slow and quiet encores, if only to calm the audience down and make them stop applauding so the orchestra can leave. Classical custom is that conductors, and soloists if any, go on and off stage ("curtain calls" it's still called though there is no curtain). The orchestra, which had stood up at the conductor's motion to share in the applause, sits down again when the conductor walks off, and by established custom can't leave until the applause ends. I've seen conductors short-circuit this by grabbing the concertmaster by the hand and dragging him or her off with them; then the rest of the orchestra can follow.
Sometimes the conductor or soloist announces the encore. Sometimes you can't hear what they're saying. Once I was reviewing and misheard what they were saying; that was embarrassing. Sometimes when reviewing I have to contact the concert management afterwards and ask. Sometimes they don't know but think they do: that, too, can be embarrassing. Rarely when a piece sounds distinctive or familiar but I don't know what it is, I hum it into a voice mail to myself and then look it up when I get home, usually in Barlow & Morgenstern's theme index.