Does moisture cause warping? I'm printing flat objects (like rectangular) with PLA on a glass bed and 70 celcius degrees (also tried 75 celcius too, 65 celcius and lower ends with adhesion problem in general). Also I use brims too. Most of the time, after a while it shrinks and warps (when print continues).

Room temperature is steady, there is no airflow to cool down things..

I tried to slow down to 20mm/sec. I tried to increase heat for first layer... Nothing helps. 

I am suspicious about moisture of the filaments. Can it be related?
Warping is caused by the plastic shrinking as it cools and inadequate bed adhesion is usually the what lets it warp. Either cleaning your print surface very thoroughly with rubbing alcohol or using something like a glue stick on your print bed will mitigate that warping enough that you won't suffer problems with your print. Printing too hot can also be a problem because the plastic will need to even cool more after it is extruded and could possibly lead to more stresses buildup in the plastic.

The dimensional stability of PLA really depends on the quality of the plastic. Storage conditions come into play as well, but it is mostly the quality of the material you need to worry about; I have some cheaper PLA that has gotten brittle due to having absorbed moisture despite being in a (albeit somewhat loosely closed) package with desiccants, and I also have a different brand of PLA that is of much higher quality that I just leave out in the open; this PLA doesn't exibit signs of moisture damage. Higher quality filaments are designed to resist moisture better and be more stable in terms of dimensions. With the cheaper brand of PLA, I have also experienced warping, but that is not due to moisture in the filament; that was actually from a new roll.

When a filament absorbs too much moisture, it can become brittle but still print. Excessive moisture will cause any water in the filament to vaporize when passing through the hotend and form bubbles that will ruin the finish quality of a print. You'll know if filament is excessively wet because you will hear quiet and sharp snapping sounds as the result of the bubbles that are formed in the plastic popping. You will also be able to see steam if you examine your hotend with a bright light as it is extruding.

I'd suggest trying a different brand of filament if possible, cleaning the print surface / adding glue, or at the very least, a new roll of filament.

(When using a glue stick to increase first layer adhesion, it could be worth noting something unusual I found; adding glue to the build plate of a Prusa i3 MK3S actually reduces bed adhesion in my experience. It might be worth playing around to see if super clean works for you, or if super sticky does. The build plate is coated in a very finely textured PEI if that is some information that could help your case.)