It's already happened on reddit, various Gaza subreddits are age gated. Anything NSFW is age gated, not just porn but things from Alcoholic Anonymous to chairs Not Fully Submerged in Water on r/chairsunderwater. XBox Live is age gated, because apparently this wasn't about preventing kids from seeing porn like they originally said but it's now about preventing children from talking to adults.
However I don't think it's to do with Palestine Action specifically. That's a separate thing, where an organisation that posed a not-insignificant military threat was dealt with in a clunky way using terrorism laws (which cover threats to civilians). I don't consider those in charge to be clever enough to set all that up in that way; rather, they'd already planned to use the Online Safety Act in this way and would have blocked anything they deemed controversial regardless.