I am a former Maps for Work/Premium Plan customer. Can/should I continue to use my Client ID?

Google hasn't announced any plans to deprecate Client ID authentication, and Maps Platform is fully backward-compatible with your existing code. But that doesn't mean you can't start using API Keys!

API Keys are a more modern authentication mechanism and provide a lot of benefits over the old Client ID model. API Keys are:

• Compatible with all Maps Platform APIs. Client IDs are not compatible with Geolocation API, Places API, Roads API, or the Maps SDKs for Android/iOS.
• Better integrated into Cloud Console reporting.
• More flexible. You can create unique API Keys for each environment and restrict them to prevent unauthorized use.
• Easier to use in server-side code. API Keys don't require digital signatures.

We highly recommend that you use API Keys for any new code you write, and that you transition any existing applications when it's convenient for your development cycle. But until then, your Client ID will continue to function for the foreseeable future.

References

Maps Platform Migration FAQs

Maps Platform Deprecation Policy

API Key Best Practices