30023 might work too. I mean, for source code storage, one could even choose 777777 to be sure not to clash with anything else.
The problem is more that the Twitter-clones and Wordpress-clones would quickly fill up with version notes. They seem to display anything Kind1 and , so everyone has to pick other numbers.
Here is the blog one: https://github.com/nostr-protocol/nips/blob/master/23.md
Discussion
Yeah, any random number.
I'm creating and posting wiki test notes, for someone else's project, and you can see them on njump. So, they do get through to the relays, they just don't get displayed in clients because of the unusual number. Which is good.