If the float property is doing its job, then use it.
Obsolete neumatica layout (div tags everywhere, instead of more semantic header, navigation, section, article, etc.), but the float is quite good.
Flex partly more convenient, but works quite differently. So don't consider it a complete replacement floatn.
By the way, you can still use inline-block. Although he has his own problems.
--- update ---
Here's the version without restrictions on the content column. However sidebar did have to limit it.