Is it bad for the bike to pedal backwards?

There are a couple of things you should consider:

1. Your bike isn't going to care. You aren't going to loosen pedals or cranks spinning in reverse, because you're not applying any load, so there is no force in which to cause precession or fretting.

2. You are getting no benefit (health-wise) from doing so. It might be better not to even bother.

Some bicycle components do have left hand threads. The left pedal and the left cup of BSA threaded bottom brackets are left hand threaded, because otherwise, they may unscrew as you pedal.

When you reverse pedal, you would only be producing minimal power. You would only have the internal resistance of the chain and freewheel to overcome. Thus, I suppose that if you pedaled in reverse for a very long time at your maximum power, you might eventually unscrew your left pedal and possibly your left BB cup (if you have a BSA BB). That seems like the only possible downside to reverse pedaling. If your components are torqued properly, I wouldn't expect this to happen at all. You would have to reverse pedal far more than people in normal cycling, at the very least.

However, you also aren’t really getting any exercise from pedaling in reverse, because you aren’t pedaling against any meaningful resistance.

No, pedalling backwards is not bad for the bike, or any of it's components in the situation you describe (an urban environment).

I base this on my own experience riding various bikes (with derailleurs, with internally geared hub) in an urban environment.

As noted in the comments on the question and answers:

• it ever so slightly increases the chance that your chain will jump of the chainrings, but in my experience you can catch this happening just in time and start pedalling forward to prevent the chain from dropping.
• don't shift gears just before pedalling backwards as that will very likely lead to your chain dropping or getting stuck

There is one benefit to you as a rider, to continue your pedal stroke while coasting.

You look like a bike which makes you safer.

I've noticed especially in ebikes that are being ridden like motorcycles, that a lack of leg motion makes the whole bike/rider combo look more static than it is.

So by continuing to revolve the crank with minimal effort, you look more like a moving object on the roadway and less like a static item, thus you're more likely to be seen by motorists.

It depends on the type of bike you have. And like RLH said also by how it was designed threadwise.

It is risky of the chain coming off. If you have a kids type bike the brakes are often operated by pedaling backwards but you cant go very far that way.

I always used to pedal backwards on corners. Once when abroad I hired a bike with a coaster brake. Therefore nearly killed myself on the first corner and trained myself out of that habit rather quickly. So possibly bad for the bike if it ends up crushed at the same time as you. I would recommend losing the habit.