Whatever the situation is in the 20%, China should just put down border posts marking the LAC and vacate. There's no reason in having a permanent presence there, especially over colder months.
There's still a small chance for there to be a peace deal so I don't see how it benefits anyone keeping this a (potentially) active theatre.
Peace deal between the two nations depends on global geopolitics and how things swing.
Also what you suggested is ridiculous. It just invites the Indians into doing the exact same thing even if only within the 20%, basically they'll just put posts to the edge of the 20% and where they patrolled. It also means inviting a breakdown of buffer agreements without even an escalation or war where buffer agreement would be broken regardless.
Yes there's no point in having strong permanent presence but strong permanent presence (outside of the 20%) is done by both and reinforced somewhat. Certainly the Indians have reinforced more than China since this is
FAR from China's major population areas and a few hours drive from India's major population areas. China cannot really mirror deploy here... not that this is required.
Within the 20% there is unknown presence. I don't call the presence permanent but both sides maintain presence and insist on it so it is sort of permanent in that sense until this issue is resolved or the border demarcated - issue resolved even further.
India refuses to demarcate because that means losing Aksai Chin. Would India agree to demarcate the border officially and renounce their claim on Aksai Chin if China gives them the 20% in exchange for finally resolving this 70 year old disagreement? Don't know. But we're nowhere near a solution from this pov. How regional geopolitics work out also unknown.