It seems that in Prefect 2.x, specifically for custom runs of deployments, you can provide custom parameter values by following these steps:
1. Go to the details page for a deployment.
2. Select "Run" then choose "Custom".
3. Under "Parameters", select "Custom".
4. Provide your custom values for any editable deployment fields.
The documentation does not mention a change in version 2.16.0 that would prevent setting a manual parameter. If you are experiencing issues with this, it could be a bug or a change in the UI that isn't documented. You may want to check the recent change logs or search GitHub issues for any related discussions.
Here's the relevant part of the documentation for creating an ad-hoc flow run with different parameter values:
Prefect Deployments.
If you require additional assistance, feel free to include more specific information about the problem you're encountering.