There's a bit of convention and practicality The only thing you really need is that your "fatal error" instruction and "syscall" instruction can be reasonably discriminated without needing to set registers at the call site. Needing to set register to identify a fatal error is not great for code size, especially in languages that generate a lot of them (memory safe languages, mostly).

Though, yes, convention does play a role. On ARMv8 you get both SVC <imm> and BRK <imm>. SVC and BRK raise different exception codes (which satisfies the "easy to distinguish requirement) but in principle you could just use BRK with a well-known immediate and eliminate the need for SVC since BRK's immediate is reported in the exception status register. And, anyways, if you have an SVC instruction and a BRK instruction, you may as well use the SVC instruction for syscalls since it's right there.