I dunno, up to the departure of Byrne, I took them at their word and assumed that a good portion of pre-Crisis stories were intact.
Most of Superboy and the Legion of Super-Heroes stories had happened--with the exception that the Superboy and the Supergirl in those stories were from a pocket universe (also pocket universe Pete Ross and Lana Lang). This was the prevailing continuity through the end of the Levtiz Legion and at the beginning of the TMK Legion. Some issues into that run, Carlin changed his mind and TMK had to throw out the continuity they were using.
Most of Justice League of America happened. Event books like LEGENDS and MILLENNIUM depended on prior JLA continuity. The exceptions being Wonder Woman, whose place was taken by Black Canary. And maybe Martian Manhunter never left or only left for a shorter time. And the JLA/JSA crossovers had to have happened under different conditions. But very little of that has much to do with Superman--so most of his adventures with the JLA could be assumed to have happened.
All of the Kirby Fourth World books were assumed to be in continuity--with the exception that Clark might not have worked for Galaxy and might only have worked for the DAILY PLANET. Hard to say, though, because you could think that WGBS was still in Superman's past, as they never said anything about it one way or the other, as far as I recall.
I know that Brainiac was rebooted--but about Mxy, I'm not so sure. Metallo was rebooted. But I assumed that other villains like Parasite and Darkseid kept their old DC continuity.
I assumed that many WORLD'S FINEST adventures could still have happened--because Batman's continuity seemed to be intact. It was only as more stories came out about Batman's past that a lot was removed from Batman's continuity. And Batman and Superman became more and more hostile toward each other as the years went by.
I would assume that the Superman-Flash race (one of them, at least) happened. Many stories with STAR labs must have happened--because Jenet Klyburn was still there. A lot of the DC COMICS PRESENTS stories could have still happened--the rest of the DC characters hadn't been rebooted so their history with Superman should have still been intact. BOOSTER GOLD was a spoiler in this, because his origins involved pre-C Superman continuity and his book is arguably the first place we see the rebooted Superman.
It's true that many stories could not have happened--or had to have happened in a different way. Anything with Luthor, Lana Lang, Wonder Woman, Supergirl, Kandor or Brainiac probably was out. But even still, that left a lot of Superman stories from the previous thirty years that could have happened.
My desire was to think that all the comics I had in my collection were still relevant. So I always assumed that all the past stories had happened in that interval of time between MOS and the Superman ongoing titles--until such time that it became impossible to make that accommodation.