I have a question so I can give a better opinion about this. Is there any technical reason to justify this proposal or is it just about personal preference? Thanks!

Examples of technical reasons: well-known third party libraries don't parse null and only ~ or in some previous YAML official spec the null value is not supported, etc.

From my point of view, this is just about personal preference. But a personal preference coming from Symfony. I first used YAML for Symfony configuration and discovered its syntax through Symfony documentation. In most pages, the tilde is used to represent null (look at the reference configuration for bundles for examples). For a long time, I didn't even know it could also be represented with the null keyword. Using the tilde is now an habit. This is why I thought @soullivaneuh idea was great to add as an option.

I don't think this is needed. Both are equivalent.