How can I add player parameters to a core oembed field for a youtube video?
I had the same problem before with a local provider. So I share the solution I used and hope it works for you too.
The Drupal core, fetch providers form https://oembed.com/providers.json automatically. So you need to change the provider to what you want.
Oembed Providers module helps to add custom providers too.


Add custom oEmbed providers via an admin user interface (providers are
stored in configuration)
Global enable/disable of providers
Modify the provider list URL (which is &gt; defaulted to
https://oembed.com/providers.json)
Disable the fetching of the
provider list (useful in an instance where only custom providers are
used)
Provides hook_oembed_providers_alter() to alter provider
definitions


So, all you need is to add a new provider for your custom provider &quot;youtu.be&quot; with your parameters too.
try it and let me know if you need more details.;)
Screenshots that shows it's working without any validation error:


what I did, was just add &quot;oembed_providers&quot; module and enabled it