I just saw the description update.
In 2.79 there was no multi-edit. These objects are 1 joined object and are indeed stretched, as would be in 2.80 if you join them too.
In multi-edit mode are separate objects and as such, there is no stretch.

So I need to join all dozens objects on my scene to adjust texel density and then separate them again because I need it to bake high into low poly ? ... It doesn't sound any good for guy who work with games- like myself. It looks like the way before I even discover Multi-edit addon for 2.79. Also preparing assets atlases in 2.80 is kinda senseless work right now.

You are right, this doesn't make sense.
I'm changing the bug status back.
This was a functionality with Multi-edit addon shipped in 2.79.

@Brecht Van Lommel (brecht) @Jeroen Bakker (jbakker)

Already looked at this. It feels a bit out of reach for me.

The only way I could thought of is,
to move faces_areas calculations
from uvedit_fill_buffer_data()
to ED_uvedit_draw_main().
But will have to pass a some variables(struct?) through draw functions.

BMFace *efa,
BMesh *bm,
float (*faces_areas)[2],
float totarea,
float totuvarea,
float Fulltotarea,
float Fulltotuvarea

I expect that you can handle it way better than this.
But if, by any chance sounds good enough, I could give it a try.

I would keep the mesh data (face area and uvarea) as much as possible in the VBO's as these will be reused. My approach would be to move the dynamic part of the data into the shader (totarea, totareauv).
this will be the total sum of all areas and the total sum of all uv area's from the objects that are being edited. The per mesh totals can be stored in the MeshBatchCache.

As I currently see it the calculation needs to take place in world space so we need to take the world_matrix into account. Currently the whole calculation is done in object local space.
I would propose to move the stretch calculation to the vertex shader. (move edit_uv_get_stretch_area to gpu_shader_2D_edituvs_stretch_vert.glsl).

Discussed last week and we will do the calculation in local/mesh space.