Sunweb can't block, because the "as though" only applies to blocking restrictions, nothing else)
Deadfall doesn't cause anything to lose forestwalk. Anything that had forestwalk still has it, so it has an ability, and Muraganda Petroglyphs doesn't work on it.
The relevant comp rule, I suppose, is:
609.4. Some effects state that a player may do something "as though" some condition were true or a creature can do something "as though" some condition were true. This applies only to the stated effect. For purposes of that effect, treat the game exactly as if the stated condition were true. For all other purposes, treat the game normally.
So when Deadfall says "Creatures with forestwalk can be blocked as though they didn't have forestwalk." it only applies to the decision of whether the creature can be blocked, nothing else, not even other continuous effects that might ultimately have an effect on whether it can be blocked. (Rusher pointed out in the comments: this is essentially rule 101.1: "Whenever a card’s text directly contradicts these rules, the card takes precedence. The card overrides only the rule that applies to that specific situation.".)
Deadfall does remove the effect of forestwalk on blocking, which does make it pretty useless, but that's not the same thing as actually removing the ability. It doesn't say "...lose forestwalk". (Example: Scarwood Hag can actually remove the forestwalk ability.) And it only allows ignoring forestwalk for the purposes of declaring blockers, nothing else. It doesn't say "resolve spells as though...", or "during the declare blockers step, treat it as though...", or anything crazy, just "can be blocked as though...". Muraganda Petroglyphs has nothing to do with blocking, so the exception doesn't apply.
For a bit more convincing, take Colossus of Akros, with rules text including:
As long as Colossus of Akros is monstrous, it has trample and can attack as though it didn't have defender.
There's a ruling as well:
Colossus of Akros doesn’t lose defender when it’s monstrous. It’s just able to attack.
I know it's defender and attacking, not forestwalk and blocking, but it's a pretty clear example of the same kind of "...as though..." templating. It really means what it says: you can ignore the ability in this case, but the ability is still there.