Hi !
I bought a Brita pitcher which has an indicator on the lid which blinks sometimes when filled and supposedly blinks when you pour (when I Pour I don’t see any blinking or lights going on )
Is it permitted to use this pitcher on shabbos / yom 
you can use it as normal.
And if it blinks each time I refill it , can I still use it ?
Yes
If the person would only drink filtered water as many people do today would there be a violation of מרקד
No it’s still ok
What about the Biur
Halacha in siman319 that says if one is a sensitive person and would never drink the water unfiltered we do not say בטלה דעתו and it is forbidden
In general we say בטלה דעתיה. The ביעור הלכה you are referring to is talking about an איסטניס who can’t bring himself to drink the water that has little pieces floating in it. Even this case is “אפשר”. I don’t think we can use that to learn anything here.