Not anymore. There is RAIDZ Expansion available. Happens on the fly.
- https://www.youtube.com/watch?v=yF2KgQGmUic
- https://arstechnica.com/gadgets/2021/06/raidz-expansion-code...
This appears to be specific to RAIDZ though, which I think I'm not using. I'm just using mirrors at the moment.
I'm incidentally currently working through the logistics for a potential upgrade and am seriously torn about whether to switch over to RAIDZ2 or stick with a couple of independent mirror pools. The "all disks thrash on rebuild after failure" property of RAIDZ2 has me scrambling for the sequential simplicity of mirrors, but... giant contiguous datasets... hmmmmmm...