It's actually the opposite now. I have never used Svelte until LLMs came around. LLMs are better at writing Svelte than React, and then you also get a faster frontend by default.

It's like Rust. Way more Python than Rust in the training data, but with LLMs you're much better off defaulting to a Rust backend and Svelte frontend than any other combination. I'd say "unless you have a specific reason", but besides legacy requirements, there is actually no reason.