ECMA standards (1978) have it that statements can be nested. It is unclear, however, if the nesting they refer to is in actual fact a restriction on the order of FOR-NEXT statements or statements themselves, and they do not provide any means to concatenate statements - however, since the mid 70s many BASICs adopted the colon character for statement separation.
But then, many BASICs have played very fast and loose with the standards as computing power increased the language evolved to make use of it (eg: PRINT command, sound commands)
However, none of the classic BASICs from the time alowed optional line numbers - that came much later when new languages moved away from BASIC towards less beginner-friendly dialects.
