By caniuse’s measure, being behind a flag is a ‘No’ for the reason I gave.
Being available as an experimental feature does not mean its release is imminent, there are ones that have been there for years. Thankfully, it’s not like they’re in a queue, some are released after only a few months.
I didn't claim this was a supported feature, just that it wasn't accurate to suggest that WebKit has no support for some of these features that seem to be missing. They are being worked on and will be turned on by default pretty soon.