What is the minimum charging current for NCR18650 li ion battery?
Li-ion batteries don't like to be exposed to charging conditions for overly long time, otherwise metal lithium build-up occurs and/or electrolyte starts to decompose. Reputable chargers (like TI BQ25896 for example) have a built-in safety timer set for 12 hours. The Panasonic NCR18650 battery specifications mention the charging conditions as "CC-CV, Std. 1925mA, 4.20V, 3.0 hrs", so a 3 hour limit seems to be preferable. 

There are not much literature on minimum charging current for Li-IOn batteries. One thing is clear that the minimum should be no less than the charge cut-off current (which is listed as 55 mA for NCR18650). On the other end, it seems like exposure to charging current longer than 12 hours is not good either. So you should set your minimum such that it finishes the charging cycle in that time frame.

In general, the electro-chemistry and solid state chemistry have many variables, and life time (level of capacity degradation) of a rechargeable battery is kind of smooth curve, where the end limits are set by marketing and business goals, or by special requirements. If you are planning to launch your product for a 20-year space mission, you probably should conduct your own research. If you are concerned with 500 mA charge to reduce your junk TP4056 charger overheating, you should be fine.