> > it was firewalled and available only to large corporations and those with personal relations to big tech execs.
> The previous call-out to IBM seems relevant: before PCs, this exact statement would've been true for (mini)computers and mainframes.
Pre-PCs, IBM mainframes actually were quite open – up until the mid-1970s, IBM released its mainframe operating systems into the public domain. On the software side, the IBM S/360 was actually a lot more open than the IBM PC was – OS/360 was public domain with publicly available source code and even design documents (logic manuals), PC-DOS was copyrighted proprietary software whose source code and design documents were only publicly released decades after it had ceased to be commercially relevant.
As we move through the 1970s, IBM became less and less open. The core OS remained in the public domain, but new features were increasingly only available in copyrighted add-ons – but IBM still shipped its customers source code, design documents, etc, for those add-ons. Finally, in 1983, IBM announced that the public domain core was being replaced by a new copyrighted version, for which it would withhold source code access from customers ("object code only", or "OCO" for short).
The main way in which IBM mainframes in the 1950s-1970s were "firewalled" was simply by being fiendishly expensive – most people's houses cost significantly less.
It is true that IBM did engage in anti-competitive business practices, but those were primarily non-technological in nature – contractual terms, pricing, etc – the kind of techniques which Thomas J. Watson Sr had mastered as an NCR sales executive in the lead-up to World War I. In fact, a big contributor to IBM becoming "less open" was the US Justice Department's 1969 anti-trust lawsuit, which led to IBM unbundling software and services from hardware–and its software culture became progressively more closed as software came to be seen as a product in its own right.