I think Let's Encrypt made a heroic effort. They deployed a hack to support Androids long abandoned by the operating system maintainer and manufacturer. If you want to blame LE for the breakage then also blame: GOOG for using the IBM PC clone business model without a device tree standard, QCOM for selling chips but very quickly cutting support, the manufacturer, and cellular carriers who prefer to lock you into another 24 month installment plan than approve an update for your existing handset.