Magento 2.0 and 2.1 support Android 4 browser:
- http://devdocs.magento.com/guides/v2.0/install-gde/system-requirements_browsers.html
- http://devdocs.magento.com/guides/v2.1/install-gde/system-requirements_browsers.html
Androd 4 browser:
- does not support the JavaScript
letkeyword: http://caniuse.com/#search=let - supports the JavaScript
constkeyword only partially: http://caniuse.com/#search=const
So, if a Magento 2 extension wants to be compliant with the Magento 2 system requirements (support Android 4 browser), then it should not use the JavaScript «let» and «const» keywords.
Android 4 browser has 8.49% market share on mobile/tabled this (2016) year:
https://www.netmarketshare.com/browser-market-share.aspx?qprid=2&qpcustomd=1&qptimeframe=Y