I personally am in favor of assuming Pressure by default.
As Karthik correctly said, because Pressure isn't actually announced in ADV formats, it would be extremely difficult to account for edge cases like Custom Game, Skill Swap, Role Play, Transform, or whatever other situations I'm not thinking of. However, we already indicate that Pressure's display may be inaccurate; it's just a matter of whether or not we want to default to assuming non-Pressure or assuming Pressure. So if we accept these edge cases will be buggy, we can meet the common needs of the vast majority of ADV games. I'm also in agreement with Karthik that hardcoding the "only Pressure Pokemon" is awkward, but don't forget we've made actual mechanics concessions to ADV in the past [
example 1] [
example 2]. I see this as acceptable to allow, since it's not changing how the Ability functions (i.e. we're not changing Pressure to actually announce itself in ADV). We'd then just modify the disclaimer message from "more" to "less":
(Pressure is not visible in Gen 3, so in certain situations, less PP may have been lost than shown here.)
So we'd hardcode Absol, Articuno, Deoxys, Dusclops, Entei, Ho-Oh, Lugia, Mewtwo, Moltres, Raikou, Suicune, and Zapdos as always having Pressure in ADV only. Aerodactyl will continue to have the same assumption of no Pressure when viewed by spectators.
This is also relevant when DCing; I was playing on the ladder and disconnected, when I reconnected the pp of the opposing pokemon showed up incorrectly.
Assuming I understand this correctly, this would just be a straight-up bug; you should always know your own current PP at all times. I can't reproduce this though.