I did some testing locally. All runs smoothly, compiled without warning.
Later on (once merged) it would be nice to write down a documentation page for the whole feature as a set next to documented individual commands. It took me a few moments to understand how this works.
I was looking how to create variable with non default value in one command, but if I understand it correctly, that's not the purpose. Variable lives in a schema with default value, but you can set it per session via LET.
Thus "CREATE VARIABLE" statement should not be usually part of "classic" queries, but it should be threatened more like TABLE or other DDL statements.
On the other side LET is there to use the variable in "classic" queries.
Does that make sense? Feel free to ping me if any help with documentation would be needed. I can try to prepare an initial variables guide once I'll ensure I do understand this feature well.
I invite any help with doc. Maybe there can be page in section advanced features