Documentation is out of date. Maybe plans to expose new api was changed?

The online documentation has only ever shown the latest released version of the API documentation.  You will not see any reference to the new features, like body battery, until 4.1 goes GA.  While it is in beta, your only available source of documentation is through your local copy of the beta SDK.

I have only noticed that something was and has disappeared (I'm pretty sure I didn't use local documentation always online version).

Understand there are some interesting things here, where with 4.0 SDKs, in the doc you'll see "Since API level 4.0.0".  You can use these today with the venu2 and venu2+ devices.  The 4.0.x are the production SDKs with 4.0.7 being the latest.

You'll see "Since API level 4.1.0" or "Since API level 3.3.0" for some things.  These are the System 5 APIs - things like body battery and stress.  Right now, there are no devices with this level of API (even if it says "since 3.3.0", they are not available on devices running ciq 4.0).  All you can do is use them with the two "system 5 preview" devices, which aren't physical devices .  4.1.0-beta1 is a beta SDK, so you can't use it to build ,iq files.

So, there's actually two different sets of docs right now.  The web based ones look to be 4.0.7 SDK docs (the production SDK), The 4.1.0-beta1 docs contained in the SDK are for 4.1.x SDK and are for the beta SDK so there will be differences.  If you are trying to use SDK 4.1/System 5 APIs you want to use the doc that's part of the SDK, and not the web based doc.

The link is to on-line documentation...

Some pages shows levels 4 API but some no.

I could see another version so I know something is wrong, 

You're seeing the doc for the 4.0.7 SDK.  The doc included with the 4.1.0-beta1 SDK has body battery, stress, and other "System 5" API calls.

In Eclipse, it's under Connect IQ>API Documentation.  In VS Code, Command Palette>MonkeyC: View Documentation>API Docs.

beta1 is just that - a beta.