That's because Group Policies are a Windows-only thing.
You can get the same (and more) control with an Enterprise subscription from Google which seems to cost about $50 per year, per device.
My guess would be that your family members got social engineered into installing that crap ("this web page only works with X, click here to install"), ort their browsers got exploited and hacked (very unlikely!). You'll probably need full MDM to prevent these websites from getting their users to enable extensions.
The problem with disabling extensions is that whatever has the capability of pushing extensions into your browser also has the ability to change the settings for addons. The only solution I can think of is to create a Chromium build that cannot run extensions at all.
And as a non windows user it took me a while to realize that this notifications come from the browser as desktop notifications and disable them. Its still a riddle for me how chrome managed to make it both very obvious and very unclear at the same time that this are websites desktop notifications. (As a counter example I used some sites which used desktop notifications on FF/Andriod instead of making a app just because notifications, that I loved)
I'm sorry that your family are... having such a hard time making reasonable choices for themselves. I have literally never seen this anywhere, or heard any coworker ever report their family rampantly adding shitty extensions. I tend to see pretty clear & obvious signals about what extensions are good & ok when I go to consume. Bad extensions seem to be discovered fairly quickly & taken down. I'm trying to imagine how folks even get to the Chrome Web Store in the first place if they have no idea what they are doing. The world to me seems no where near as grimdark as you project.
Alas I think it requires a paid Google Enterprise account, but your family sounds like their need external management of their browsers. That they should, like a school computer, have an administrator & a denylist or perhaps even allowlist of what extensions they can use.
This post spreads so much Fear Uncertainty and Doubt. Trying to justifying ending a good thing because some creative user keeps finding a way to misuse, to not listen to sense, to not make good judgement... I find it unfortunate that such heavy fearmongering, such terror at the world is allowed to sway us so heavily.
Ultimately I want 3rd party sites hosting extensions. Not Google. And I want moderation teams able to surface claims that some extensions are bad. We need more choice, more democracy, more ability to help each other. Sunlight is the best disinfectant. Simply giving in to the bed-wetting terror of, oh no, freedom & denying ourselves user-agency is intellectual suicide for the web.
It doesn't sound like you think your family is at all educateable in any way, you seem to think this is a horrible lost cause & that we must withdraw power & good for the world to protect your vulnerable unfortunate tribe. You're using that as a weapon against the world, against good, against freedom, against capabilities.
This is extremely menacing a position you've made, using your own family's purported victimization a weapon against good.