Despite the fact that you can actually kill Myrkul for good over the course of the MotB campaign by eating his lingering soul, according to Wizards, we canonically did not do that, thus leaving him clinging to the edge of existence until he was revived after the second sundering, which actually revived several gods, caused others to be subsumed into greater deities, and caused the creation of several more. Th second sundering was wild and basically served as an excuse for Wizards to pull a reset on certain events.
The wall still exists as of 5e, but the only reference you'll find about it is in the Sword Coast Adventurer's Guide, which is also where they cover some of the gods returning or vanishing. Technically Myrkul, Kelemvor, and Jergal can all exist simultaneously as they cover slightly different portfolios. Kelemvor is god of the dead, and arbiter of the dead; Myrkul is the god of death but also more broadly, endings; and Jergal is the scribe of the dead and archivist of the libraries in the city of the dead. This distinction is thin, but important, otherwise one of them is going to get subsumed into the other, and at this point I think Kelemvor has the highest rank, deity-wise.
It's entirely possible that someone like Bane or Myrkul would give up a portion of their god-hood in order to gain freedom from their own portfolios. Being a god gives you a lot of power, but also places a lot of restrictions on you, this is how Ao intended it to be. Being a powerful mortal or demi-god allows you more freedom, as you aren't confined to a portfolio or at the mercy of belief like a full god.
