Reserved slots aren't included in ULX 3
Plans aren't concrete, but it might be included in a future release.
I've been using +sv_visiiblemaxplayers as a kind of work around.
If you can normally host, say, 10 people (if you can, I'm jealous of your upstream), you would set maxplayers to 10, sv_visiblemaxplayers to (10 minus the slots you want 'reserved')
Once the visiblemaxplayers number has been reached, no one else can join through the main server list.
They can however connect through console "connect yourservername.com" or "connect your_ip". If you shared that with your only your admins, they can join.
When I run a server, its a dedicated on my local lan, so I usually set visiblemaxplayers one higher, because when I play on my server, I don't take up the external bandwidth.