It does suppress testosterone but once you stop using it , the testosterone will go up on its own. That it is why SARMs tend to be used in a cylical way.
There are very limited researches on SARM especially on possible side effects from a long term basis. I did come across a couple of research papers where the SARMs benefited older people and those with cancer in clinical trials.

Steroids, however, have known established long term side effects, which can be debilitating.

Are you a doctor?
Or have you done proper peer reviewed research on those who take steroids responsibly and followed them up long term?
Or do you have any peer reviewed published research to back up your claim that responsible use of steroids do not cause long term side effects?