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.

It was 1996 for me. I forget where the original SSH (SSH1 protocol) came from, but I do remember compiling it on a Slackware box around that time.