Yes they were. IIRC at least one of the major manuals of style endorsed them at least in some situations.

I've been curious about this sort of thing, but I haven't been able to find good info on it. For example, it seems to me that the format of "It's not x, it's y" is widely accepted. I'm not sure if that's by simple common use or by way of some nuance that I'm not aware of. Certainly in some cases it can be difficult to avoid both comma splices and sounding stilted. I'd love to know more -- I don't seem to recall anything about it from when I had a copy of the Chicago Manual of Style.

No one accepts "It's not x, it's y" as correct. Well, maybe some illiterate Philistines do. But no one really thinks that's correct.

You have to put a semicolon in place of the comma or it is just plain wrong.

That’s interesting. Maybe it’s considered hyper correct?

Which style manual are you referring to?

Don't remember. This was 30 or 35 years ago.

There was a long tradition in English and maybe also in other European languages of choosing punctuation depending on how long a pause a speaker would take at the point, with a comma denoting a shorter pause than a semicolon, which in turn denoted a shorter pause than a colon, which in turn denoted a shorter pause than a period -- instead of using punctuation to convey some semantic distinction as most writers do today. The comma splice might be a vestige of that tradition.

But the pauses are semantic in a way. Those concepts cannot be well separated. Not a linguist, but I can tell pauses are used by speakers and listeners for a variety of reasons, linked with both semantics and delivery -- to separate parts, to give and demarcate "processing time" for speakers and listeners, to create and satisfy expectation, etc. The cadence of speech is semantic (though its semantics are not of a simple propositional logic type).