NI has the option to leave the UK only because they were given that option by the UK under the GFA. The Minsk Agreement does not contain any such provision. Hence any declaration of independence by the DPR and LPR is meaningless, just like the declaration of independence by the Confederate States of America or any potential declaration of independence by Belgorod Oblast.

Treaties work exactly as I described, the UK even considered reneging on the GFA as part of Brexit. Nevertheless they are not worthless, because a country that does not honor its treaties will find itself diplomatically isolated, unable to form trade agreements, etc. No country wants to end up like North Korea.