Since OpenSSH 8.8 (2021-09-26), RSA signatures using the SHA-1 hash algorithm are disabled. So by default, you cannot SSH into RouterOS branch 6.x with a certificate using an OpenSSH client newer than 8.8.
Will you make a further upgrade to the SSH service in the v.6.x branch?
MT staff (I think it was @normis) clearly stated, that ROS v6 is feature-frozen, it’ll receive only (some?) security fixes.
Support for ellyptic cipher algorithms is IMO not security issue. After all, OpenSSH did not discontinue support for legacy algorithms, they were deprecated … meaning they are dissbled by default but ordinary user can enable them … preferrably per remote host.