I have tagged v4.5.0 but I can't publish it to the Registry yet.
In the meantime I think you can add protobuf to spago.yaml dependencies like this:
package:
dependencies:
- protobuf
workspace:
packageSet:
registry: 60.5.1
extraPackages:
protobuf:
git: https://github.com/rowtype-yoga/purescript-protobuf.git
ref: v4.5.0
subdir: library