It's not currently legal for all. At the present moment whether blocking a constituent is legal depends on which circuit you live in.
> While these cases address only issues related to an individual's explicit blocking of other individuals, I can't help wondering if a future case might limit an individual's freedom to choose which instance to use. If SCOTUS rules against blocking, might individuals with government roles be barred from using instances that provide instance-level blocking or either individuals or other instances?
In a certain sense anything can happen. But there's no reason to think this is more likely than outlawing apple pie. The current case is about whether elected officials can click the block button when logged into official accounts for certain reasons.
Note that platforms are still allowed to block for any reason regardless of the outcome in this case. Note also this case leaves open the possibility it might be permissible for an official to block constituents who broke neutral rules. There is nothing here to suggest any platform changes would be needed.