I think Mesa Mark II was the first truly modern high gain amp built, and that came out in 1980. Modern levels of gain on tap with all intended distortion generated in the preamp while the poweramp was designed to run clean, built-in ability to cut lows at the input, and extremely flexible EQ.
If you consider the other extreme of "modern" guitar tone, which would be hyper-clean and compressed tone with digital ambient reverb, that was available in the late 70's, so only a couple years before modern high gain tone.
So yeah, I think "modern" guitar tone has existed in some form or another for around 40+ years. Sure it's been iterated and improved upon since then, but the basic foundation of "modern" tone has existed for about about that long.
If you ask me, the biggest paradigm shifts in guitar since then have mostly centered around taking existing tones and making as many of them as possible as instantly available to players as possible through switching systems, and these days, the game is to make all those tones available in smaller and lighter packages, either through smaller amps, elaborate pedalboards and pedal platform amp rigs, or digital modeling.
At this point, I don't know if it's possible to take modern guitar tone to a completely different place without doing something as drastic as converting it into a MIDI controller or something and fundamentally changing what it is at its core.
I don't think this is a bad thing, however. Nobody ever demands the sound of a violin to be fundamentally altered in service of some kind of "advancement" of the instrument, and it's been popular for 500+ years now. Historically speaking, the guitar is a relatively "new" instrument but I do think it's reaching a kind of evolutionary maturity now. However, I also think it's already tonally varied enough to be relevant for as long as people keep making music.