~
Alright, I spun up a new Debian 10 Droplet.
(1) Updated package manager and packages
sudo apt update && sudo apt upgrade
(2) Install latest protobuf for Debian 10
sudo apt install protobuf-compiler
(3) Check protoc version
root@helloworld:~# protoc --version
libprotoc 3.6.1
==> Indeed, seems like Debian 10 Buster
does not have latest protobuf in its package manager by default! So it's not your fault, BlackHatCoiner!

Now, last Raspibolt release is quite a while ago, so upgrading it is probably not an option. Let's solve this otherwise.
[1] Add the
buster-backports sources to our package manager.
echo "deb http://deb.debian.org/debian buster-backports main" >> /etc/apt/sources.list.d/sources.list
[2] Update package manager
sudo apt update
[3] Install protobuf again
sudo apt install -t buster-backports protobuf-compiler
[4] Maybe, for good measure also install these (they should come with protobuf-compiler, though).
sudo apt install -t buster-backports libprotoc-dev libprotoc23