Telegram's "TL" serialization, that's part of its network protocol, also uses a bitmap for optional fields. It's an okay protocol overall. The only problem is that the official documentation[1] was written by Nikolay Durov who is a mathematician. He just loves to overgeneralize everything to a ridiculous degree and spend two screens worth of outstandingly obtuse text to say what amounts to, for example, "some types have type IDs before them and some don't because the type is obvious form the schema".

[1] https://core.telegram.org/mtproto