It probably accepts other key types and it's just the UI that's outdated. I doubt they're using an SSH implementation other than Dropbear or OpenSSH, and both support ed25519.
The problem is not the RSA math itself but that it is both extremely slow and implementing it is particularly susceptible to bugs and side channel attacks
https://blog.trailofbits.com/2019/07/08/fuck-rsa/
Most of the situations I encounter RSA are in projects where I hope RSA is implemented correctly. I have a lot of Let's Encrypt certs that are still RSA and my main SSH keys are still RSA. All of these were generated quite some time ago. I understand the problem with projects that implement it incorrectly but I'd hope OpenSSH and certbot aren't those projects 😥