Firstly, thank you all for your answers.
These are Sil-Pads or another equivalent brand thermally conductive insulators. They are used because they take less labor to install than grease, and provide more electrical insulation. In the past, metallic foils were also used. The performance is worse than using grease by itself with properly lapped surfaces.
However, if there is a reason that the heatsink and CPU can't be allowed to touch electrically, it may not be safe to use grease alone.
It makes a lot of sense but it's not a thermal pad (like the ones used in GPU chips in order to keep them cool and physically high). It's more like senso suggests....
No, that is just plastic with a strong adesive, its there to protect the exposed pads and caps on the top of the cpu carrier.
Use Artic MX-4 or Kryonaut, no no-name thermal paste..
Totally agree it is more like plastic tape (a bit of elastic if you pull it off). That's my concern, to not using it and get a short or similar problems with the pads and the heatsink but I think it wouldn't be a problem i don't use it (my bare CPU chip looks like the second pic). BTW, I am using Nocta-H1 over the CPU and GPU crystals...
Just leave it there. It is not deteriorated. it is not even because the surface under it is not flat as there are ceramic capacitors scattered on top of that CPU.
It is really deteriorated believe me and it doesn't stick anymore; many years of maintenance you know... (the pic I attached it is not of my CPU because my Laptop is assembled right now so I dind't want to open it again at this moment and I wanted to show you how the material looks)
The thing is before I created this thread I replace the motherboard (because the soldered GPU needed some rework but I bought a brand new (OEM) one from a trusted supplier and cheap). So I reused the CPU & the other components from my original one and replaced that material with kapton tape (without touching the crystal which has the grease over it), I clean the laptop with compressed air/Isopropyl and re-assembled it again and turn it on... all seems correct... 10 min later it turn itself off (no temperature problems neither on the GPU or CPU; 48C and 45C respectively. So Thought it could be that material... or the pressure applied by the heatsink... or probably the motherboard needs a BIOS update (because cames new from the factory). However, I had that doubt about the materrial too so asked