Maxima does something like this. It's weird to use. I think := for assignment makes more sense if you want = for equality (i.e. like Pascal etc.).

Golang uses : to assign in struct initialization and := for new variable assignments so there is some prior art.

Practically speaking I find : to be challenging to quickly scan out of source visually. In golang this objection is muted slightly because struct initialization usually ends up being clearly indented by the formatter.