There seems to be no upper limit; I tried over 1000 entries and they caused some significant lag when re-creating the empty channel on rejoin. Adding over 100,000 or even multiple million entries will take some time but might really cause a DoS affecting the whole network. Once the list is set up, to cause disruption, all needed is to /part and /join again. And again. And again. And again. :/
There seems to be no upper limit; I tried over 1000 entries and they caused some significant lag when re-creating the empty channel on rejoin. Adding over 100,000 or even multiple million entries will take some time but might really cause a DoS affecting the whole network. Once the list is set up, to cause disruption, all needed is to /part and /join again. And again. And again. And again. :/