Enhancement type MOSFETs are much more common than depletion type. They are available in both N and P channel, so this is no help. To be sure, look at the data sheet. The gate threshold voltage VGS(th) is positive (for N type) so it is enhancement type.
*Specifically, the gate threshold voltage VGS(th) is the same polarity as the active Drain voltage: V(BR)DSS so it's enhancement mode.
Enhancement-type MOSFETs are "normally off", until a positive voltage (N-channel) or a negative voltage (P-channel) is applied to the gate.
Depletion-type MOSFETs are like JFETs, they're "normally on", and you need to provide an opposite voltage to the gate (negative for N-channel, positive for P-channel) relative to the source to turn the transistor off.
Those are all rated to ≥500V, is there anything rated to 50V? Obviously they can be used at lower voltages but 330mΩ channel resistance is too high for low voltage battery applications.
It could be useful as a source follower in a linear regulator or audio amplifier.
With an enhancement device you have to take the gate above the positive supply to turn it on, with a depletion, you don't.
The other option is to use a p-channel device which is more expense, has poorer characteristics and can be hard to stabilise when used as a common source amplifier driving and unknown and potentially capacitive load.
So far the only depletion power MOSFETs I've seen are those linked by jpanhalt which are high voltage devices. I wonder why one would want a high voltage normally on switch? It sounds like a safety thing but you can't rely on a semiconductor for safety critical switching.
As far as I know all JFETS are inherently depletion mode, so that's the only kind of JFET you can buy.
So far the only depletion power MOSFETs I've seen are those linked by jpanhalt which are high voltage devices. I wonder why one would want a high voltage normally on switch?
I don't understand the problem of having a maximum Vds of 500V. They are spec'd to less than 5V Vds and the packages (e.g., TO-252 and TO-268) are common for mosfets rated at 20A. The other producer I found, Supertex, may have smaller packages, as it does not offer products with quite the current capability of the IXYS devices. Perhaps, the high voltage rating is simply a function of the way they are manufactured.