I read somewhere that some of the Clans emptied out the last of their caches. I think it was after the Wolf/Falcon War but I don't remember for sure. It was in reference to Invading Clans though.
I would think the less successful Clans would have already done so. If they hadn't been emptied before the Reevings they would be after.
Also isn't extinct mostly for purchasing and repair purposes? As in you can't go down to the local Mech emporium and order an extinct unit. However that doesn't preclude, rare museum exibits, barn finds, periphery finds, miss labeled crates, etc. And if you some how obtained one keeping it running will be very difficult. Basically, as far as anyone knows its extinct but you really never know for sure.