Portfolios are still mentioned in the Player's Handbook and Dungeon Master's Guide
The concept of taking a god's portfolio as a mortal has in the past only happened during extraordinary circumstances. Before the timeline of 5th Edition there was the Era of Upheaval, caused by Ao casting down the gods to walk the world in mortal forms in response to the Tablets of Fate being stolen (they were eventually returned but Ao destroyed them). This time is when a lot of gods were killed. This era ended with Ao restoring and rewriting the Tablets of Fate, writing down the names and purposes of all the gods. With this the pantheon of gods appears rather set in stone as long as they gain worship.
I do not know of any instances of a god's portfolio being taken by a mortal during the timeline of 5th edition. If you are a DM you are of course free to introduce similar cosmic events or perhaps forgotten rituals that would allow your player's to ascend. If you are a player, all I can say is ask your DM.
This section is specifically about the adventure Rime of the Frostmaiden since you mentioned it in your question. If you are not a DM do not read this, it is a spoiler and you should ask your DM about this.
The adventure is set around the premise of Auril casting some sort of divine spell around Icewind Dale and the adventure explains that "each casting of the spell leaves the Frostmaiden weakened, with just enough divine power left to barricade the mountain pass with blizzards and churn the Sea of Moving Ice with blistering winds". Auril is thus set up as being drained of most of her divine powers and the adventure provides stat blocks with the intend of your players fighting and possibly defeating her. However, Auril is not mortal as described in the Epilogue: Auril’s death is temporary, for a god who has mortal worshipers can’t truly die. When the Frostmaiden is resurrected on the next winter solstice, she has all her divine power.