"would be required on a per user basis or track the access rights along with the content, which is infeasible and does not scale"
Citation needed.
Most enterprise (homegrown or not) search engine products have to do this, and have been able to do it effectively at scale, for decades at this point.
This is a very well known and well-solved problem, and the solutions are very directly applicable to the products you list.
It is, as they say, a simple matter of implementation - if they don't offer it, it's because they haven't had the engineering time and/or customer need to do it.
Citation needed.
Most enterprise (homegrown or not) search engine products have to do this, and have been able to do it effectively at scale, for decades at this point.
This is a very well known and well-solved problem, and the solutions are very directly applicable to the products you list.
It is, as they say, a simple matter of implementation - if they don't offer it, it's because they haven't had the engineering time and/or customer need to do it.
Not because it doesn't scale.