Not sure what GP means but I gather the x230 era (2012?) has a cult following. I picked one up a few years ago when a laptop died and I didn't have the cash for something new: it is still my daily driver and I'm not replacing it til it dies.

By contrast, I know someone who got a T480 second hand and it lasted six months. My guess is the 2012 era was when the change happened

It's been a gradual shift, with a few obvious changes along the way.

Among a few: The keyboard switch from the old 7-row (whose pinnacle was at the x220/T420 era with double-height esc and del) to the new 6-row (with later ever decreasing key travel) to the current x9 (which is basically just a yoga keyboard with no trackpoint, no key grouping, and the loss of pgup/pgdn). Things like the modular battery options vanished. The case got flimsier over time with e.g. the magnesium rollcage first vanishing from the display, then from the base. (And no - from enterprise experience - the carbon fiber composite isn't generally "as good or better", esp. for failure modes like punctual force on the display. Or...grabbing the laptop by the display and using it to fan your BBQ, which doesn't faze my old X41 :) ).