Imposter syndrome for scientists isn't weird. The pressure upon scientists nowadays have probably led to unprecedented levels of imposter syndrome though.
There have been massive shifts in expectation, including managing science by quantifying output (and putting targets on output numbers). On top of that, the world has become much more competitive - scientific output increases exponentially, as Lodinn pointed out in their comment. That points to a lessened effect in earlier times.
On the other hand, the nature of a scientist is not to be sure, but to question their knowledge continuously. That points to a still-existing effect in earlier times.
Overall, I'm sure imposter syndrome in science has been around ever since it was called science. I would also argue that the changes since the 80s/90s/2000s have led to a much more industrialised and competitive approach to science, in which imposter syndrome would be much more common.
(1) The environment changed
You wonder about how things were in the 70s and 80s. I would argue that that is not a fair comparison, given how the world has changed. The world is massively more connected right now, and there have been foundational shifts in how academia is governed - at least, in some countries, but I wager in most. Case in point: full professors used to be appointed by the Queen in The Netherlands. That made it impossible for their host institution to fire them - you cannot overrule the Queen. So they got rid of that rule, somewhere in the 80s I believe.
More pertinent to your question: there has been an enormous increase on managing quantifiable aspects of research. I am not positive when that started. Likely, it varies from country to country - my guess for Netherlands (where I know the academic system reasonably well) is that this started in the 90s. I know of a full professor sent on early retirement due to lack of publications in mid-90s at least.
Again in The Netherlands, managing academia by quantification received an enormous boost around the end of the 2000s. The Minister of Education reapportioned a large part of science funding from institutional funding to project-based funding.
Before that change, research groups in NL had funding to appoint PhD students and postdocs themselves. Afterwards, they only had funding for permanent staff; money for temporary positions was contingent upon project acquisition. This led to a very competitive environment - not just for grants, but also for staff positions. Universities started setting quota's and expectations on scientific output and (later, after the former was being gamed) on scientific impact.
Such changes lead to increased pressures on PhD students. E.g.: the better the PhD student performs, the better the project turns out, the easier to attract further grants, the more secure your job, the better your chances for promotion, etc.
Now that's all a Dutch perspective; nevertheless I wager that similar processes occurred elsewhere. For example, Peter Higgs (after whom the elementary particle was named that was found by the LHC) said in an interview that he'd probably been fired for lack of output if he hadn't been nominated for a Nobel prize (in the Guardian).
(2) The nature of science is to question knowledge
A second aspect is that the nature of science is to question knowledge. A famous example of not doing this happened in 189 by famous scientist Lord Kelvin, when he said "There is nothing new to be discovered in physics now. All that remains is more and more precise measurement." He lived to regret his words with the advent of General Relativity and Quantum Mechanics...
Side bar aside: scientists are trained to question knowledge - even established knowledge. Basically, the internal monologue "is that truly so? If so: show me the proof!" should be ingrained into a scientist early in their career. It is not at all weird to then apply that to other aspects than merely the research one is performing. That can easily lead to questioning the validity of one's career. The tough part is that while there is likely evidence that someone is a decent scientist, that is not the same as conclusive proof. Add to that the hyper-competitive world we find ourselves in nowadays, both within academia and without, on social media, and it is not weird for anyone's best available evidence against imposter syndrome not to be convincing to a scientist whose basic nature it is to question if the evidence is sufficient.
I cannot give you a complete account of how the academic world functioned in the 70s/80s. However, in the 70s and 80s, far less papers were published. Moreover, universities weren't (at least, not everywhere) run as if only offering a desk for permanent staff to set up their own grant-attracting shop. While it is in the nature of scientists to doubt (and thus: doubt their own value), I do think the societal and managerial changes since the 70s have changed science careers such that they continuously have to validate their own existence nowadays.
In short: Imposter syndrome for scientists isn't weird. The pressure upon scientists nowadays have probably led to unprecedented levels of imposter syndrome though.