This hasn’t been said elsewhere, so just a note for everyone else who’s been curious about it: In order to import Ed25519 private keys, they appear to need to be in PKCS8 format.
ssh-keygen doesn’t have the ability to convert Ed25519 keys yet, so until it does, you can convert it with it with the npm (JavaScript) package sshpk:
npm install -g sshpk
sshpk-conv -T openssh -t pkcs8 -p -f ~/.ssh/id_ed25519
and take the output of that, and you’ll be able to import it into RouterOS 7.15rc1
Hope that helps.