So the case we have here is this sign combination: https://imgur.com/VsiC1py . This is definitely not how it's usually (and logically) done, but that's what we have.

The relevant subsection from the article would be https://wiki.openstreetmap.org/wiki/Key:maxspeed#Vehicles

In this case, there is no signed speed limit for "general traffic" for the primary `maxspeed`. The speed limit for general traffic here is 50, implied by default urban speed limit. Then the signs set the *exceptions* for specific transport modes to 90 and 70.

Now, is the sign combination here stupid and does the 70/90 limit cover 99.9% of practical cases? Yes. But technically it does not cover bicycles, quads/tricycles, mopeds, horse carriages, electric scooters, different types of psvs or any other legally-distinct transport modes that those supplementary signs don't apply to. That is the result of car-centrically signing exceptions like this - they will not apply to all cases. So we also reflect that into `maxspeed` subtags.

If the signs were instead more logically set like this similar to your example: https://imgur.com/x1kuE61 , then, yes, it would be `maxspeed=90` + `maxspeed:hgv/bus=70` because the top sign would refer to everyone, i.e. "general traffic", and the 70 is an exception.

You didn't link anything in your last message, so I'm not sure which area you are referring to. I am sure there are still many errors and mistakes with max speed tagging, but I would have to see the exact example to say more. If there is only 70 sign and nothing else, then yes, it would be just `maxspeed=70` for everyone.