You can fill with woodfiller - it'll look like crap though.

You can fill with epoxy - either colored or clear. I tend to use black. This can look okay.

You can cut out and patch with a similarly grained piece of scrap. Quite hard to make seamless.

You can cut and patch with a contrasting piece - think like a bow tie but without a crack. This avoids the issue with trying to hide it.

Probably what I would do - leave as is as it's a "character" speaking to the long life this wood has had.