That's probably true, although I really don't know why it's particularly relevant. GoFundMe probably doesn't want people funding anti-vaccine campaigns regardless of how they arrived at the web page.
> 2) The above makes me think such campaigns will simply move to a GoFundMe competitor and be equally (un?)successful, effectively nullifying any changes
How does that nullify any changes? There are a finite number of crowdfunding platforms, and this reduces the number by one. And of course, this is a particularly large and prominent one.
> 3) Gives more attention + victim points to anti vaxxers
I don't really buy the "don't give them what they want" argument. If they really do want press about how harmful their practices are and how children are dying because of their campaigns, well okay, I'm fine with giving them this desire. I think that they should be "victims" of de-platforming, even if that's what they want. It's theoretically possible to get the Streisand effect, where their platform is increased, but I highly doubt that's likely when you remove them from huge platforms with massive market share, like YouTube or GoFundMe.
> 4) If their discourse is further pushed out of mainstream platforms, does it actually reduce the spread of their ideas or just limit their exposure to counter-arguments and public condemnation?
There are probably some anti-vaccers who are genuinely mislead, but the prominent voices in the movement almost certainly are aware of the invalidity of their claims. I don't think we have any lack of exposure to pro-vaccine arguments.