So I was thinking about changing loot-based XP to a system that requires a bit more exploration.
I decided to look at the delves in 2020 so see when people did any exploration.
The last time anyone received 1+ xp for exploration was for Icy Gate I.
Prior to that, it was the Lost City 11 and Lost City 12.
Prior to that, "Ape Gate."
The last time a group found a new significant location in Felltower, and actually went there (Wizard Eye doesn't count) was Session 126, back in January.
So, five sessions of exploration in 2020 out of fifteen - 1/3 - had enough exploration to warrant 1 xp. None had enough to warrant 2 xp worth (admittedly a notably higher bar.) Of those, two were past two gates, and two were in the Lost City finally going into two places the group had eyeballed in the past but never penetrated. Only one involved delving deeper into Felltower.
That's fine as a proportion, really - the gates are meant for exploration. So is the Lost City (not that there is much left to do). But it's still a fact that 2/3 of the sessions didn't result in exploration, wether because they were extended multi-session combats, clean-up delves in already explored areas, or otherwise moving about in already-discovered areas.
If, generously, you assume that all multi-session delves that involved some exploration in a session count as exploration in both, it's more like 8/15. Much better . . . but a good chunk of the game is about exploring for loot. It's not like the other seven were spent exploiting loot discovered in other delves. It was just attempting to get some (Draugr), or hoping for some (most of the rest.)
So best case count, half of the delves just don't involve exploration. And only 1/15th involed poking down just a little deeper into Felltower. I won't delve into the numbers for now, but sessions with exploration more often than not come with 2 or 4 xp for loot, and more often 4 than 2. So the data do seem to support the idea behind that rule change musing.