Activation Lock for stolen iPhone with passcode and Find My off

No. Unfortunately in that case, the thief can bypass the passcode by connecting your iPhone to a computer and erasing it. If Find My was enabled, after that it would present the Activation Lock screen.

rehman221 wrote:

My stolen iPhone still has my Apple ID signed in, which I never logged out from.

In this situation, if someone tries to restore or use the iPhone, will the Activation Lock be triggered, or would it be possible to use the device without entering my Apple ID?

Even if the iPhone had your Apple Account signed in to iCloud Services, it would still need Find My explicitly enabled beforehand.

Refer to What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support where it says, "sign in to iCloud.com/find"

and see if your device is signed in. I was kind of under the impression that unless you specifically disabled Find My that it was automatically enabled when you set up a device. If you are signed in then set the device to erase but do not remove it from your account. If you do that then a finder can make use of your device.

Read this support article to learn what you can do if Find My iPhone or Find My iPad isn't enabled on your missing device --> How to find your lost iPhone or iPad - Apple Support

rehman221 wrote:

Then what can I do

Unfortunately there isn't much you can do. Why didn't you have Find My enabled in the first place? If so, you could've tracked it and prevented anyone from using it. I advise you do that on your next iPhone.