I realize this question may have a different answer for each field of computing (enterprise vs. business vs. personal vs. field equipment), but there should still be an objective point in time where this transition took place for each field of computing, and that's the goal of this question, knowing when that time was and what the events surrounding it looked like.
Back in the early days of (at least personal) computing, it seems like security wasn't much of a concern at all. Pretty much everything ran as "administrator" (or whatever the equivalent was for the machine in question), applications could read and write to wherever they wanted in RAM, a single buggy application could crash the entire system (looking at cooperative multitasking, yuck), and data was usually transmitted unencrypted. Even some systems designed to be somewhat multi-user (CP/M in particular) lacked all security - one could just switch to someone else's user and have full access to their stuff. The closest thing I can see to security in these old days of computing (at least for personal computing) was some simplistic forms of DRM (like the NES used for region locking). And even that didn't work particularly well.
Contrast this with modern-day computing, where well nigh everything that gets transmitted over the Internet is encrypted so strongly that even the most powerful computers known today can't break into it in a reasonable time span, security vulnerabilities are a big deal, systems are designed for multiple users and have privilege isolation, etc., etc. Even CPUs may have extra security features built into them (like Intel's SGX). It's become just part of how computing works.
I've not used enough old computers to know for sure, but AFAICT, in Windows, the transition from pure functionality-based computing to security-based computing happened somewhere around the time of Windows 2000, which had Administrator, Power User, and User security levels (IIRC), and an entire page in the Help manual on why using an Administrator account all the time was a bad idea. (Contrast this with Windows 95, which only had a username and password for logging you into the network, and would just give you total admin privileges to the physical system if you clicked "Cancel"...) I also remember reading about HTTPS and the now-obsolete S-HTTP in a very old HTML book (like, CSS was competing with DSSSL, old).
When did security really start becoming "on the radar" in the world of computing, and why? Was there ever a transition like this in the world of business computing (servers and mainframes), or have those always been security-oriented? What were some of the first security features, and what inspired their creation?
Edit: Also just realized another core part of the equation here - CPUs used to just be made faster and faster as technology and innovation would allow... which ultimately led to the mess we now know as Spectre/Meltdown (to the best of my knowledge). Now we have to be careful with these kind of things, oftentimes accepting dramatic performance cuts in so doing.