Astral Projection is a high level spell because it makes the entire party virtually unkillable. In this context, the projections are fully functioning duplicates, and if a duplicate gets killed, the projector simply returns to the original body. It is a powerful spell.
On the other hand, projecting ones mind outofbody, is a low level spell. First, it is an inconvenience to secure the meditating body, while the mind is wandering. Second, according to the folklore, if the mental projection gets injured, the body gets injured in the same way. For example, if a projection encounters an other projection, and one punches the other in the eye, then the original body spontaneously evidences a black eye. In other words, hit points are hit points.
At a low level, outofbody is relatively vulnerable method of Clairvoyance. Probably spell level 2 (compare Beast Sense). However, the outofbody mind can interact with the physical world as the virtual hands are made out of telekinetic force (compare Mage Hand), being ether, and can even manifest visibly the same way a ghost can.
The outofbody mind can also interact telepathically with other nearby minds, including mental enchantment spells. Reciprocally, a person in the material world can fight back against an outofbody mind by force damage and psychic damage. Similarly, if the mind is interacting with a physical object, it is possible to interact with that object in away that can injure the mind − thus the original body.
Spells relating to Protection from Evil and Good: because their 5e versions are moreso protection from extraplanar creatures, they *should* block out all outofbody minds, the same way it blocks out extraplanar creatures.
These Protections spells should be a simple way to prevent all forms of clairvoyance, ethereal traveling, and so on. The heroes can use these to defend themselves. More importantly, the DM should be able to use these spells as a simple low-level method of plot protection to prevent heroes from scrying or extradimentionally entering certain rooms. In this way, the DM can keep certain things secret, and make the heroes work to discover the mystery.
For example, the DM can have a villain cast Hallow on an important room, a Protection related spell at spell level 5, thus by this simple tool, permanently keep away all extraplanar scrying, ethereal travel, outofbody wandering, or so on.
Because there would be such a simple way to keep out ethereal creatures, it becomes possible to have spells like Etherealness and Clairvoyance, Outofbody, and ‘phasing’ thru solid objects, at the lowest spell levels. The DM can still keep certain secrets secret.
