Opt-out philanthropy would have a very similar effect that taxation does in a socialist society. The differences are that individuals can legally choose whether or not to pay and can also choose who to donate to under opt-out philanthropy.

Those are two big differences, but otherwise I think you're right. In other places of "Life You Can Save", Peter Singer talks about opt-out philanthropy in the form of an additional 1% income tax automatically donated to a GiveWell top charity, unless you change either the target or opt-out completely.

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you can download the national/state/local budget and an opt-out form every year, and fill out the opt-out form for each line item in the budget that you don't want to donate toward. [...] Democratic socialism approximates such a system, but by turning line items on and off for everyone at the same time. That may not be optimal.

I don't think democratic socialism approximates the system, because no actual (legal) opt-outing takes place. Secondly, my intuitions would be that such a system for all taxes would be disastrous in the case that I predict it would massively lower the government budget and make funding programs completely unpredictable.