It was not mandatory. In case of India, Virat Kohli asked the BCCI management for permission to knee for BLM, which was granted by the president Sourav Ganguly. It has nothing to do with the ICC directives. During yesterday's match between Namibia and Afghanistan, both the teams refused to knee. Namibian players held up their hands in support, while Afghanistan did the right thing by completely ignoring it. I still don't understand what Asian teams have to do with the BLM movement. Afghanistan and Pakistan are doing the right thing by standing against this drama.
This issue was very much settled but it came to cricket in this WC. Just like sometimes cricket promoting awareness on breast cancer. Don't worry this moment won't pop up in other icc tournaments. Even if it pops up I don't see any harm in kneeling down for BLM.