no one can tell what either '+' or '|' does in this context without guessing, because it does do neither "arithmetic addition", nor "concatenation", nor "union set" (or bitwise op, or any other op one could come up). '+' is more familiar or '|' might even be completely new for some users, but this distinction does not help making '+' in this context "more obvious".

What I can only say for sure is that it is an operation on two dicts and the result is possibly again a dict. So consulting the doc will be needed in either case.
Union set, while being commutative, is not a manifestation of the "addition idea", but of something else - a union set idea :), which is different from the addition idea, because it is not only more specialized, but, and this is the important difference, requires recognition of another concept - identity, which the addition does not need, and on the abstract (idea) level operates differently (vaguely saying, "addition" preserves the integrity, "union set" preserves the identity).
The dict merge (or update) operation is unfortunately related neither to all "addition manifestations" nor to "union set" in a sense that it comes from the same idea, but it is an idea on its own, because apart from the recognition of identity (which union set requires), it also requires a concept of association (which neither addition nor union set use or need) and the actual operation is again different from the former two.

Using either '|' or '+' is technically not correct, if we want to have it consistent on the abstract level. But '|' (the idea of what '|' is representing) is more similar to what is happening in dict merge than the addition idea. Since introducing a new (operator) symbol for that seems to be unacceptable (for practical reasons), '|' is the best candidate we have.

Thanks for the nice run-down on operators and logic, and I mostly agree.  As you say, neither is accurate due to the different mathematical structure of dictionaries as compared to sets, etc.  To me '+' would seem the most natural in the sense of that is what I would have tried first, as many others have stated.  From the "feeling" of it, '+' points toward 'addition', 'what comes later'.  I see it as rather dangerous to use '|' union because it behaves similar - as it is not the same - confusion may be larger.  Yes, you have to look up in the doc what it does - or, for the lazy, do a hand full of tests with examples (I often do that).

+1 for '+'