02673853 Its it still ok to use the workspace feature from a security standpoint if one wants to have 3 spaces (personal running simultaneously?
Yes it works fine. And always has. The only real concern is the fact that whatever manager app you use will be given a lot of high level access to your system (at least in the workspace I don't entirely remember about outside of it. Either way point remains largely the same.) so you will have no choice but to trust the developer of said app to not ship a malicious update and you will also have to trust that the app doesn't get exploited. This is definitely more of a threat model thing to consider though.
02673853 It seems like the feature is a bit outdated
Not quite sure what you mean by that.
02673853 and the last update of the shelter app (https://f-droid.org/packages/net.typeblog.shelter/) was 2 years ago
I don't think this would directly affect the security of the workspace itself much since the workspace security is mainly handled by the system but considering how much access a workspace manager app would need... Yeah I could see why you would be concerned about that.
You may want to consider using something like insular instead cause it largely does the same thing and AFAIK is still receiving updates.
But yes overall to your original question the workspace feature pretty much works as well as it always did and coexists just fine with the private space. You should be able to do what you have in mind with little to no trouble.