[ljm]

What makes the letter U obscene?

[Am4TIfIsER0ppos]

You can make the word fuck with it. That upsets children on the internet.

[account-5]

I doubt that upsets any children on the internet; more likely it upsets some adults on behalf of children on the internet.

[thaumasiotes]

If that's what you're trying to avoid, it will be a lot more effective to remove F.

[ljm]

Might as well go for Base27 then. Strip out all of the vowels and you can't accidentally make naughty words any more.