As for the selected answer, bare in mind several things.
- Depending on the type of modern site, the powershell method will remove all buttons at once.
- For those cases it doesn't hide (just user voice, but not the mobile) you would have to do decide what to do: deal with customers who might understand this is not an opt-out option or look for an alternative.
You can go the CSS way but do not go with a string selector, Get mobile app will only work for english speakers, all other users are left behind. Besides, going string selector is creating a strong coupling with translationss you don't have control over.
I you have no other choice but to go with CSS I suggest you create an SPFx extension, at least this will apply to all modern pages and you can even link the extension to a site design and make it a deployment step with any new site. Check site desing for SharePoint online and SPFx extensions for more information.
I know this is a lot harder, but a better SharePoint IS possible :)