Eric, there are 3 overlapping terms to keep in mind (and the distinctions aren't always clear, nor used with consistency from one chipper to another:
1. Retired - a chip not available on the tables or at the cashiers cage (in jurisdictions where chips may be sold at the cage). These chips have been pulled from play, although a stray can always show up on the tables or in the cage from time to time. These chips can (and have) occasionally come back onto the tables. They have not been "de-monitized", or called in for redemption by a deadline date.
2. Obsolete - chips called in for redemption, after which the will no longer be cashed. For instance, the Lady Luck has called for redemption certain LE's. After the deadline date passed, these chips would be obsolete, even though the casino remains open. BTW, some chippers used the terms retired and obsolete interchangeably, figuring that if the chip can't be obtained (i.e. "current") then it's obsolete; they don't use the "retired" distinction.
3. Closed - A casino, after it closes & redemption period passed. These chips should be classified as closed. It doesn't make them automatically rare; in fact, if the rack gets out, the chips may be available below face value, especially for the $25+ value chips. In cases of ownership changes where the name doesn't change (for example, prior ownerships of the Sahara-Las Vegas), these chips may be called "obsolete" instead of closed, since the casino remains open under other ownership. The prior chips were de-monitized, however.

Unfortunately, there's not a lot of precision, and some overlap in terminology, among some chippers regarding these terms.

Hope I've made that perfectly muddy for you <g>!