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The usual alternative is a committee, see for example Apache Software Foundation's Project Management Committee Guide: https://www.apache.org/dev/pmc.html

I suspect an informal grouping (i.e. more long-standing/senior contributors with default control over some parts of the codebase and operating by informal consensus between themselves) is probably at least as common (maybe more so) than an officially named committee with rules of procedure.

The main difference between a CTO and a BDFL is that if you disagree with a BDFL, you can just fork the project under a new name. This right to fork is key to making FLOSS development work. If you disagree with a CTO, there isn't much that you can do besides buying up the whole company (see Elon Musk and TWTR as an example).

The most inefficient decision-making structure known, democracy and voting?