Heart-rate based training (and even racing for ultra-distance events) was all the rage in triathlon starting in the late 1980s. I started racing triathlons in 1985 at age 24 and had some success at the local and regional level by the late 1980s. Since many triathlon pros at that time were advocates of heart rate monitors, I bought one. It didn't help me. I never raced any faster than I did without using a heart rate monitor.
I'm 60 now and haven't used a heart rate monitor for about 25 years. So I have no idea what my heart rate does during a typical ride. OK, that's not quite true. I know when it's near maximum and I know when it's in "zone 1". It's just that I don't know what the precise BPM for those zones are for me :^)