In generally, airplane luggage compartments are pressurized but not at sea level. It’s generally assumed that the under-floor luggage compartment area is pressured to 8,000 feet above sea level, which is enough to push corks out of wine bottles bottled at sea level. So while it’s better than being unpressurized, especially when cruising at 30,000 feet, it’s certainly not the same as being at sea level.
Another, similar concern, is temperature. Luggage compartments are almost never temperature controlled. It’s generally recommended that you keep sensitive medication on your person during travel (especially if you need it and in the event the airline loses your luggage).
There are some cases, especially in smaller plans, where the luggage compartments are pressurized the same as the cabin itself. In these smaller planes, there’s no separate luggage area and so luggage is stored in another part of the airplane. The smaller the plane, the more likely this will be the case.
When it doubt, check with the airline to see if the compartment is pressurized or temperature controlled. It will depend on the model of your aircraft and only your airline will know the specific details.