Each of the big servers implement their own extensions, or share some extensions but not others. All poorly recorded in many documents across the Web.

To have a modern irc client, it is far from a clean protocol. If you just want the basics, it will still connect and send messages.

I tried to implement an IRC client in about 2004 and no popular server was implementing RFC 2812, which was released in 2000. I realise that's a long time ago but things move slowly in the IRC world, so I would be surprised if things had changed a lot since then.

RFC2812 was codifying one particular network's protocol choices and was never accepted by the wider community of IRC server and client developers.

There have been widely implemented improvements (eg RPL_ISUPPORT / 005) which succeeded by discussion and consensus rather than trying to lay down a new standard by fiat.

Yes, you can definitely connect manually through telnet (I did it back in the old days). The only problem is it will not stay connected too much, because you should also reply to server's PINGs at regular time intervals otherwise you'll get disconnected.

With most servers you don't actually have to specifically reply to the PINGs, just sending something regularly should keep you connected.