According to the spec, colon not followed by flow indicators ([]{},) should be considered as part of the key and not throw an exception which is gonna be the 4.0 behavior according to the deprecation.

For example, foo:{foo:bar} should be valid but it won't be in the current state.

BTW we can also consider fixing other inconsistensies:

• [] is allowed in sf mapping values but not in the spec {fo[o: bar}
• {} is allowed in sf sequence values but not in the spec [fo{o}: quz]
• mapping values not followed by : (i.e. {value}) is allowed and should be converted to {value: null} (i fixed this one in [Yaml] Base the parser on a StringReader #19750, in 51b87f7)

WDYT?

cc @xabbuh