This isn't supplanting parenting duties, it's giving parents the tools to actually parent. Without this your options are basically no YouTube access at all or full YouTube access with zero granularity in-between. The default for logged-out people has to be restrictive or else it's trivial to bypass. The current rules for porn, alcohol, and mature games are a joke and have been since their inception. Everyone being born on Jan 1 1901 is a meme at this point. It's checking a regulatory box without actually doing anything and if YouTube tried it they would be raked over the coals.

Libraries do restrict the materials that can be checked out by minors and I doubt that counted as supplanting parenting. How else could you give your kid any level of independence without the cooperation of other people?

The restriction previously happened at a device level, moving it to google seems like an elaborate justification to move the default to restrictive and break external video viewers and apps like yt-dl.

You should control your kids device and make restrictions there, not place them by default on the entire internet.