S-1 Thermal Noise Removal necessary

I need to extract the backscattering coefficient. Is it necessary to extract the thermal noise from the images before March 2018 of Sentinel-1? And to those after that date?

If you want to remove the additive noise you shall apply thermal noise removal operator and you can apply it to images before March and after March 2018. The only difference is that the algorithm for thermal denoising is implemented in different way for products generated since IPF version 2.9 which include additionally to the denoising annotation in range also the one in azimuth.
For you as an user it shouldn’t make any difference to use SNAP thermal denoising operator for products generated before and after March 2018

Thank you! I use python