booting ESXi from SD cards is not supported for vSAN hosts more than 512GB RAM.
Is there a limitation for booting "normal" ESXi hosts from SD cards with more than 512GB RAM?
that doesn't seem to be true anymore. According to Hardware Requirements for vSAN you CAN boot a vSAN host from an SD card even if it has more that 512 GB RAM.
It's just the default size of the core dump partition that is then not sufficient to hold a complete dump. Thus the recommendation is to extend the core dump partition (if sufficient free space is available) or use a network core dump collector instead (this is what I do with my vSAN hosts).
For non-vSAN hosts the default core dump partition is large enough for a RAM size of up to 1 TB. When going beyond that the same recommendations apply.