The NodeJS v10 upgrade only affects Functions deployed via the Serverless API. Functions deployed via the UI still run on NodeJS v8. The only way to leverage NodeJS v10 is via the new deployment API.
All new Serverless API customers (after Sept 24th) run NodeJS v10 by default.
Existing serverless customers will be be transitioned to NodeJS v10 on November 4th 2019. At this time, newly deployed Functions and redeploys of existing Functions will run on the NodeJS v10. If you want your account to be upgraded sooner, you can contact Twilio support.
Note: Functions must be be redeployed to take advantage of the new version. The runtime of currently deployed Functions will not automatically upgrade.