For an epidemic to occur, you need not just a lab leak, but a population sufficiently naive to the pathogen. H1N1 was displaced by H2N2 in the late 1950's pandemic, which in turn was displaced by H3N2 in the late 1960s pandemic. Thus it hit the cohort of people aged 25-6 or less who'd never been exposed to H1N1.
I found an NIH article that says the likelier origin is that the 1950 virus was used to produce a weakened live virus vaccine candidate that lead to the reemergence and not an accidental leak. It also concludes by saying there has never been a likely lab leak epidemic ever observed.
A small number of SARS infections in 2003-2004 are also believed to have been due to laboratory accidents [2].
This article [3] gives an introduction to the subject from the perspective of a journalist who has reported on laboratory safety in the US.
This article [4] published in Nature in January 2012 by members of the US National Science Advisory Board for Biosecurity reviews the risk of a release of an engineered form of H5N1 influenza. It includes some alarming remarks such as:
'We found the potential risk of public harm to be of unusually high magnitude' and;
'A pandemic, or the deliberate release of a transmissible highly pathogenic influenza A/H5N1 virus, would be an unimaginable catastrophe for which the world is currently inadequately prepared'
The authors take the possibility of release of a dangerous pathogen from a laboratory seriously, though the article is prospective rather than retrospective.
[1] https://en.wikipedia.org/wiki/2007_United_Kingdom_foot-and-m...
[2] https://en.wikipedia.org/wiki/Severe_acute_respiratory_syndr...
[3] https://eu.usatoday.com/in-depth/opinion/2021/03/22/why-covi...
Those details do inform some details of the correct policy response. For example, they determine the relative importance of better PPE at the bench vs. better QA before allowing the vaccine to leave the lab. They don't change the overall question of whether scientific research has ever caused a pandemic, though. That causality is what matters, not whether the sign on the door said "lab" vs. "experimental vaccine nurse".
For example, if the pandemic originated from a WIV researcher who became infected in the field (during their many expeditions to remote bat caves that no other humans would routinely enter), was that a "lab leak"? Literally no, since they weren't in the lab. The causality would still be the same, though--if not for that scientific research, that virus would likely have never left the cave.
To avoid such confusion, it's probably better to say something like "unnatural origin", or "origin arising from scientific research". A much bigger mouthful than "lab leak", though.
The article's abstract opens with the statement 'The 1977-1978 influenza epidemic was probably not a natural event'.