steps on a treadmill

I'm guessing you meant "walking" on a treadmill. If so, you just select indoor walk as the activity in the Activity app, and it will count the steps.

I have an Apple Watch 2 and iPhone X. Both were in same room, with wifi. I ran on the treadmill, used indoor walking activity and zero steps were captured in my Health app.

Posting here is not going to get Apple support to know it didn't work for you. This is a user to user support forum and Apple does not participate here.

To get back to the question though, when you were running on the treadmill, were you actively swinging your arms, or were you holding onto the treadmill. I ask, because if you were holding the treadmill and not swinging your arms, nor using the Activity App to set it for indoor exercise, it would cause an issue for the Watch counting steps.

Get the most accurate measurements using your Apple Watch – Apple Support

Calibrating your Apple Watch for improved Workout and Activity accuracy – Apple Support

Update your Apple Watch – Apple Support

If you run on the treadmill then select the "Indoor Run" workout type.

I use it all the time, although paused now because of the Y is closed due to the pandemic. I would do a 3mph brisk walk for one hour, and used indoor walk and it counted my steps just fine.

I'm happy for those of you that it works for. I'm wanting Apple Support to know it did not work for me with my watch, my phone. Could be some sort of update that needed.

You ask about an update, what version of WatchOS is on your Watch?