Nahal's Reckless Dweomer has been directly incorporated into the Sorcerer's class features.
Starting in 3.5E, the spell was converted into the "Reckless Dweomer" class feature for the Wild Mage prestige class:
Complete Arcane, Page 68
Reckless Dweomer (Su): At 9th level and higher, a wild mage knows how to spontaneously convert her own spell energy into random, unpredictable results. As a standard action, she can eliminate a prepared spell or spell slot of at least 1st level to create an effect similar to that of activating a rod of wonder. The character's student of chaos ability (see above) applies when she uses her reckless dweomer ability. See page 237 of the Dungeon Master's Guide for details on the rod of wonder.
5th Edition goes a step further and makes all of a Wild Magic Sorcerer's 1st-level and higher spells function this way.
PHB page 103, Sorcerer, Wild Magic, Wild Magic Surge
Starting when you choose this origin at 1st level, your spellcasting can unleash surges of untamed magic. Immediately after you cast a sorcerer spell of 1st level or higher, the DM can have you roll a d20. If you roll a 1, roll on the Wild Magic Surge table to create a random magical effect.
There are a few good reasons for the changes.
- Being a class ability rather than a spell, ensures that the ability makes sense within the context of being a Wild Mage, rather than say a Thief using "Spell Thief" to steal the spell, or other strange spell cross overs.
- The wild surge can be determined by the DM in cases where the player and DM don't want wild surge to happen all the time.
- Because of the way spell casting works differently in 5e, it would be game breaking if you were allowed to cast a 5th level spell slot spell, using only your first level spell slot.