Command "generate-votes" is not defined I'd like to use Drush 10.3.6 to generate votes on my development server.
Based on these instructions for a previous Drush version (8.x), there is some kind of module/extension for this.
drush generate-votes

However, when I try it, it doesn't work for me. The error message I get is the following one.

Command &quot;generate-votes&quot; is not defined.

Has this feature been removed or deprecated? Is there any other way I can generate votes from the command line?
This command doesn't come from Drush core but from the Voting API module. This command simply doesn't exist yet in the currently latest Voting API release: 8.x-3.0-beta2.  See, the directory /src/Commands where Drush commands since Drush version 9 get registered now simply isn't there yet.
You need to get the latest Voting API dev:
$ composer require 'drupal/votingapi:3.x-dev@dev'

There the /src/Commands directory contains a VotingApiCommands.php file with all the commands in it. The module needs to be enabled to use them, of course.

Voting API version 3.0-beta2 only contains the legacy votingapi.drush.inc file for Drush commands up to Drush version 8.