FYI: Related to #19374 (A massive PR that actually adds this feature)
One could pick out some good from that PR to implement this feature.

A good way to exclude fields is to use the annotation Group. See documentation here: https://symfony.com/doc/current/serializer.html#using-serialization-groups-annotations

Is there any problem with this feature or should this issue be closed?

Closing here for now. Please leave a comment if the suggested groups do not solve your use case.

The idea was I wanted to designate fields as not serializable by default (as in, not in the Default group).

Groups are a nice feature, but if I go that way, I have to tag all the other properties of the object (and probably its relations) with a specific group, which seems like a huge overkill when you just want to exclude a couple fields from default serialization.

Thank you for this suggestion.
There has not been a lot of activity here for a while. Would you still like to see this feature?

Friendly reminder that this issue exists. If I don't hear anything I'll close this.

I've added the "help wanted" label because I find this feature really useful.

@derrabus I think the @Ignore attribute introduced in 5.1 should be enough.

https://symfony.com/blog/new-in-symfony-5-1-serializer-improvements#added-ignore-annotation

Oh, indeed! Let‘s close this issue then!

@derrabus I think the @Ignore attribute introduced in 5.1 should be enough.

https://symfony.com/blog/new-in-symfony-5-1-serializer-improvements#added-ignore-annotation

Looks like same is achievable with attribute: https://symfony.com/doc/current/components/serializer.html#ignoring-attributes