When did nested quotes become a thing?
The forum has handled them fine from the beginning.
That said, the "strip nested quotes" option is turned on so when you click the quote button it only inserts the top level of quotes into your post. This is to prevent argument threads from quadrupling in length as everyone quotes each other (without reading each other).
However, you can still quote a couple people & edit where the [quote][/quote] tags end up.