If the abscess blew out a while ago (as your post implies), new hoof wall should be gorwing downward, and new sole should be filling in, but sole growth is usually quite a bit slower than hoof wall, and if it was a particularly deep abscess it may take a while to fill in. However, if it is no longer cuasing lameness or oozing pus, the wound has sealed itslef off, and isnt' likely to become reinfected,unless in muddy/foul conditions or the original source of irritation isn't removed.
A shoe can TRAP dirt, but it doesn't really matter, either way. Once the abscess is done, it's done, if it popped on it's own. If cut into to make it drain, the chances of reinfection are much higher.
Old drainage holes don't pose a threat, usually and will eventually fill in with new, healthy growth, or in the case of the "Graveled" abscess, the wall simply grows down and the hole is trimmed out or worn off at some point.