This appears to be valid in scripts (but not in modules):

let?.x

Playground

Certainly Acorn, Espree, and TS-ESLint can all parse it:

• Acorn AST explorer
• Espree AST explorer
• TS-ESLint AST explorer

We seem to be able to parse let.x, just not let?.x.

Discovered this oddity in ESLint's test cases.