They are actually 99% of the time stickers, not paint. If removed, they oftentimes leave a "ghost" number where the aluminum has oxidized around them or the sticker glue has pitted the aluminum behind them. It can be lessened and sometimes totally removed through polishing, but that is a big chore to take on - unless that is really the look you want. So a lot of folks, just leave the "ghosts of past owners" showing like a badge of honor.
If/when you are planning on joining the WBCCI you can call to see if that number (or one close to it) is available, in which case you can hide the ghost-numbers. If you don't join and you leave them on, don't be surprised if some other club members come up to strike up a conversation ~