Places to ask questions or discuss leadership topics with others.
Rands Leadership Slack - Started by Michael Lopp (aka Rands). See the Welcome post for instructions on getting an invite.
Software Lead Weekly Slack - Started by Oren Ellenbogen as an offshoot of his Software Lead Weekly mailing list. Instructions on getting an invite come in the newsletter.
Ask a Manager - Discussions on a variety of management topics, each triggered by a question from a reader.
Manager Tools - A portion of the Manager Tools site that allows for forum discussions.
Reddit CS Career Questions - Many questions revolve around direct development career options, but some involve management.