It's r-values that cannot be assigned to. What's really missing from most conventional math, I think, is the notion of conditional expressions based on equality testing. "=" is usually only used in a declarative way, to declare that two expressions are equal.

See n+k patterns in Haskell for a programming exception to the rule you mention.

Actually, though, set-builder notation can use the equals sign to express a condition. e.g. the examples here https://en.wikipedia.org/wiki/Set-builder_notation. Maybe there are other examples in math.