Your phone keeps working, but it gets riskier over time. Monthly security patches stop, so any new flaw found after that date stays unfixed. The safest move is to switch to a phone that’s still supported.
Custom software is an option for tinkerers, with limits. Installing a third-party version of Android, such as the privacy-focused GrapheneOS or the widely used LineageOS, can extend an old Pixel’s life, but neither can fully protect it, because they no longer get the manufacturer’s own firmware, kernel and hardware code. LineageOS, which still supports the original 2016 Pixel, requires an unlocked bootloader (the startup lock that verifies the system is untouched), and that can stop banking apps, mobile wallets and some games from working.