I'm a big fan of threaded conversations. After getting used to them, you start realizing what a terrible mess unthreaded chat systems such as Slack are. They are just horribly unproductive.
FlowDock is the only team chat system I know that has threads done right. Unfortunately, it has two flaws: 1. It has not seen any development in years. The app has changed ownership recently, so this might change for the better. 2. It is stupidly easy to misuse by newbies who are not yet familiar with threads: instead of replying to threads, they just keep starting new ones and mess up conversations. Teaching everyone to use threads in FlowDock is hard.
For this reason, I cannot recommend FlowDock to a team of 66, especially if it includes non-technical staff. Unless you're eager to patiently work with every individual teammate to constantly remind them to not start new threads.
I'm submitting a feature request to FlowDock asking to solve this problem for good.
The second best is Microsoft Teams. Its threads are not nearly as efficient as FlowDock, but they exist and they do their job. And it does not suffer from the newbie misuse problem.
Don't use Slack. Slack claims to have threads, but its threads implementation is a patch on top of unthreaded conversations, and it actually makes things worse. Slack is counterproductive.
Discord is the only chat system I know that is perfect for open and semi-open (invite-only) communities. There's also Gitter but it hasn't seen progress in years. Discord has no threads and no benefits for a closed team.