IRC daemons aren't saving PRIVMSGs and other events to disk. They send them to the target user/channel and then that memory is freed. This comparison makes no sense.
Interesting! +H wasn't a thing when I stopped using IRC/writing bots. InspIRCD (and its amusing default config file) was the daemon I always used. Looks like my client of choice (irssi) never added support for this feature.
InspIRCd's +H does not depend on client support; when configured it sends the history on join to all clients (except those which marked themselves as bot with mode +B). What irssi lacked though is the "server-time" extension, which means it would display the wrong timestamp for history messages.
