In The Elder Scrolls III: Morrowind, players had access to (and in some places required) Levitation spells.
In Oblivion, Levitation spells have been removed. The reason levitation was removed, according to the developers, is that, unlike in Morrowind, cities are located in separate cells from the rest of the world. Since the cities aren't fully rendered until you enter them, it wouldn't look right if you were to fly over the walls. Levitation also was sometimes used as an exploit in previous games of the series, so it was removed to preserve game balance. However, by climbing on rooftops you can in fact exit towns and see the unrendered world.
Two NPC conversations that you can overhear in the Imperial City mention that Dovyn Aren has been discouraged by the passage of the Levitation Act. This is presumably an effort to establish continuity with Morrowind. The specific quotes are:
- "He still teaches, though he lost his passion for it after the Levitation Act was passed. Can't say I blame him."
- "He's getting older, but he can still teach a bit about Alteration. He's been teaching it since before the Levitation Act of 421."
Also, one of M'aiq the Liar's quotes is about levitation:
- "Levitation is for fools. Why would we want to levitate? Once you are up high, there is nowhere to go but down."
Several areas incorporate this limitation into their level design. The planes of Oblivion, including the Sigil Keep, is one notable example. In many multi-level dungeons, there is also often a door at the end of the last level which leads back to a walkway above or near the entrance, which you can jump down from for a quick exit. Having a levitation spell available would eliminate the need to traverse much of these areas (although this can often still be achieved with high enough Acrobatics).
While the date of 3E 421 actually predates Morrowind's start date of 3E 427, the province of Morrowind is not subject to Imperial laws due to joining the Empire by treaty rather than conquest, allowing otherwise illegal practices (such as slavery).