I think that it also depends of the juridiction. In the UK when you refurbish an old building you can go to "building control" where there will often have microfiched structural drawings that you can consult. Nevertheless you aren't allowed to make a copy of the drawings without authorisation from the original designer. And if the original designer went bust, that's no excuse for not obtaining permission. On one hand it can be a PITA to have to spend several hours writing down what you see on the drawings but it is also nice to know that your hard work is protected.
Given I work as a civil and structural engineer in teh UK, I can confidently state that nothin in teh above response is correct. There is no UK 'building control' which holds structural drawings for all buildings. If you're lucky the owner of the building may have some drawings, but probably not. Some councils have a central depository of drawings of buildings they own, but again its hit and miss what they have and whats been lost.
If someone does have drawings, they are invariably the property of the owner of that building, to do with as they please. As standard in UK (and every other part of the world I've worked in) for building and structural design contracts the finished design becomes wholly the property of the client.
They archive old submissions of structural calculations for building controls (https://www.istructe...ing-Control.pdf - guidance from Istructr-E on this). Some of these include detailed drawings, especially older ones, I once found some gems really neatly drawn by hand. If you ask nicely, they will let you access the drawings, I've never managed to make a copy though. In most places where I have worked, there was a disclaimer at the bottom of the drawings saying no copying without permission otherwise copyright infringement.
At least this is how it was working in the late noughties when I was there.
You were very lucky in the local authorities you dealt with. Very few (and none I've yet worked with) archive anything beyond the statutory minimum time. And what they do have rarely includes full structural drawings - as you'll see from the IStructE guidance this level of detail doesn't need to be provided for building regs approval - depending on the complexity of the building, you can get away with a few pages of calculations and some hand sketches. You just need to provide sufficient information to prove sufficiency of design.
But the lack of copying is nothing to do with authority from the designer, as the IP doesn't belong to them. You need the permission of the owner of the building at that time. If you have that, they'll happily provide you with copies (at a cost, of course)