In the Linux / BSD world, SSH took off incredibly fast for the time. I'd estimate that maybe 80% of people had moved to it within the first year of its release.

But adoption stalled when the original SSH moved to a commercial license in 1996-ish - many of us stuck with the last free version, but vulnerabilities started to pile up. There were various half-working alternatives, but it wasn't until OpenSSH came out in 1999 that the remaining telnet holdouts started to move across.

I worked for an ISP in the mid-90s and had been on the Internet since 1989 or so. I recall the progression for me was something like this:

We used telnet in college no problem. It was a fairly well-accepted method of remote access. The heterogeneous network had many different modes, but a major dialup point was the Annex box, which supported telnet into the Unix or VMS machines.

Between Unix machines, we would often prefer "rlogin" instead. There were several horrific iterations of other remote-access protocols such as "remsh". rlogin was notorious for its "/etc/hosts.equiv" authorization method which trusted DNS and should've been perceived as Swiss Cheese from the outset. rlogin was, IIRC, directly related to rsh and rcp and used the same frameworks. rlogin was no more secure than telnet, but probably less secure because of its conveniences.

We also used port 23/tcp for remote management, for example Cisco routers. They weren't running telnetd, but it was the port where you connected remotely and logged in with or without credentials.

rlogin persisted alongside telnet, until encryption came into fashion and ssh was distributed. Once ssh was available and working well, everyone knew that telnetd and rlogind were on borrowed time. The services were shut down and disabled in inetd. The ports were sometimes blocked. Security advisories went out.

I suppose it took a long, long time for ssh to finally dominate, and for people to abandon telnetd mostly, but it was fairly thorough. We all recognized the superiority of sshd's authentication and encrypted channels.

There were mitigations for people to extend their legacy use of telnetd and rlogind. For example, tcp wrappers and fail2ban could be implemented. Firewall filters could select only authorized networks. VPNs could tunnel through an Intranet that still used them. So, the services lived on wherever they didn't need to be exposed on the public Internet. But I think most Unix admins got the picture by the end of the dot-com bubble.

I'm a fan of 'nc' / netcat for this purpose. It's small, quick, and can send or receive over TCP or UDP.