~ $ cat ~/.ssh/id_rsa
cat: /data/data/com.termux/files/home/.ssh/id_rsa: No such file or directory
~ $ cat ~/.ssh/id_ed25519
cat: /data/data/com.termux/files/home/.ssh/id_ed25519: No such file or directory
It probably accepts other key types and it's just the UI that's outdated.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/
~ $ cat ~/.ssh/id_rsa
cat: /home/dos/.ssh/id_rsa: No such file or directory
~ $ cat ~/.ssh/id_ed25519
cat: /home/dos/.ssh/id_ed25519: No such file or directory