battery discharge faster

Open support ticket. This was raised already in beta versions before but wasn't fixed before firmware went public so now we just have to bother Garmin support with this issue.

How do you even disable it? Garmin Share is an activity like the rest? It uses battery only if you open up it. You need to open it to get or send anything. 
Blaming Garmin Share feels just plain stupid. 

Someone said during beta that disabling it helped, many others said it had no effect, me included. 

You disable it via Connectivity menu in watch. 

You turn it off in the “Connectivity” menu.

The battery drain started after the feature was released.

That's why it might be the offensor, and why people are trying to disable it.

Interesting that it's there. What is even the point of that setting? It just hides the app? 

Using Garmin Share | Garmin Customer Support 

Mentions nothing about this.. and to receive or send you need to start it. I don't think that disable does anything else that just hides it from the activities.

For every firmware release there are people complaining about battery drain. 

Garmin share does not run in the background in anyway based on any documentation there is. How could it take battery? 

If you could receive files without the need of starting the Garmin Share, then this might be different story.

In any case, 0.8% battery drain per hour (18%/day) is no imagination but a fact with this FW. 

I doubt that it is a fact for everyone. Like the OP said it was from 3% to 5%. You say 18%. Pretty huge difference here. It might be a fact for you, but OP had different percentage. I haven't keep count on mine....