Can unplugged guitars and basses be heard through apartment walls?

"can be heard" by who? Maybe the people for you house have a sensitive ear for a Sonic 2000 Ear Amplifier commercial from 1990's. Maybe they lost 30% of their auditive sense. Or maybe their ears are totally ok. Only they can answer your doubt, not the appartment.

Download any sound metering app for you smartphone and do some measurements on a quiet day. Those apps simply use your phone's mic and display some kind of decibel gauge or frequency response graph. Measure yourself talking on a normal tone, or a normal conversation or for example the t.v. on news broadcast at your usual listening volume.

Then measure your instruments playing as you normally would (Both plugged and unplugged).

That readings will take out the guesswork. You can also use the meter while playing at night just to be extra sure.

Just remember to hold your phone at the same distance from any sound source you measure, say, a meter away from you, a meter away from your tv speaker, a meter away from your guitar, etc.

I seriously doubt an unplugged guitar or bass will be any louder than a normal conversation, a measurement will tell you for sure.

I'm no musician bu I do own an electro acoustic guitar with nylon strings. It cannot overpower a conversation unless playing really hard, and surely cannot be heard through my apartment outer walls (Inner walls are drywall)

In my apartment I have tested with my music player and at a level that would overpower normal conversation in the bedroom, is barely noticeable if I go to the hallway outside the apartment and close the door.

If by any chance you still think you may annoy your neighbor with your playing, there are several good DIY sound treatment How-to in youtube and other platforms. Many are oriented towards echo and reverb cancellation for better sound recording, bur the working principle (Absorbing sound energy) is also good for reducing the amount of sound that escapes a room. The cheapest one I've seen uses a lot of tightly packed towels mounted on wooden frames and covered with an aesthetically pleasing fabric.

One problem is resonance with anything the guitar is touching. If you are sitting on a soft surface and the instrument is only touching you, then the most they can possibly hear will be somewhat less than what you can hear. If part of the guitar is touching a bedpost or a wall or floor (note double bass players!) then the sound will carry through the whole building.

A neighbour inclined to complain will hear ANYTHING through the most solid of walls. But it shouldn't annoy a tolerant one.

The short answer to your question is YES, it is possible to hear unplugged electric guitars through walls. Even a solid body will make enough noise to penetrate some walls. Floors I am not sure about but that depends on several factors.

1. The quality of materials and construction used in the apartment.

2. How hard you play.

3. Where in the apartment you are sitting and how the instrument is aimed.

Back in college I lived in an old apartment near school that was a 3 flat converted in to a 6 flat. The dry wall and frame separating two flats on the same floor was extremely flimsy. You could push on it and it would move. You could hear conversations through the wall. You could even hear the scratching sound of people writing while they studied. And I did get a complaint about playing an electric guitar unplugged from the downstairs neighbor. If I were you I'd communicate with your neighbors, be friendly. And see if maybe you could do a test. If you know roughly where their bedroom is (assuming that they'd be asleep when you play) then just make sure you are on the opposite end of your apartment.