Does Plesk support RockyLinux as an alternative to CentOS 8?
Yes, Plesk supports RockyLinux OS started from Plesk Obsidian 18.0.41.
The conversion from CentOS 8 with Plesk to RockyLinux is supported, as well.
In-place conversion of CentOS 8 to RockyLinux is a recommended way to move from CentOS 8 (which is going to be EOL at the end of 2021 by OS vendor). To convert, follow instructions from here.