Well, I'll try summarize answers and my experience.

At beginning, Internet used network classes, because of hardware limitations (later switched to address blocks). And even in 1990s still existed very old hardware, only could use class addresses.

What classes mean, existed early very large organizations, got more addresses than they could use. And even happen few cases, when such organizations lost rights for these addresses.

And these unlucky organizations was some big whales, like IBM or ATT/Bell or Sun.

And once invented solution - state some big enough network as not allocated to use under NAT (or when network is not connected to Internet). So, departments of big organizations could use TCP/IP stack in their networks, even with old hardware, but don't need to contact Internet officials to got real internet addresses.

192.168 was just first C-class network prefix, was not assigned at the moment (or just released).

Later, to list of unassigned added 172.16/12 network.

Note, the CIDR RFC didn't come out until Sep 1993. Thus even brand new network equipment in the mid 1990's were still very classful. And even then, knowledge of how to properly use /etc/netmasks in SunOS v4.x (or the equivalent if some other network stack even had one) was very scarce.

In the mid 90's, SMBs connecting to the Internet would have very typically obtained a /24 from their ISP, and had direct connection online, no firewalls, barely any proxy servers (although that was popular for some mid sized customers that would have needed multiple /24s or even a /16 to get all their workstations online).

It wasn't until the company Network Translation, with the PIX came about that anybody even considered doing private IP address in general as a firewall strategy with NAT translation using private IPs. And then it took years and years to become popular. Long bought by Cisco at that point.

I don't think Cisco IOS even had NAT until something like 10.2, when it was a premium license package.