for jobs - indeed is better or other small avenues in their heyday such as HN who is hiring (all my jobs have come through hn)
other avenues - local slack channels.
linkedIn - good for initial connection with strangers you don't know and might find valuable
linkedIn - good for keeping tabs on companies or new startups
try https://corvi.careers I been building it purely as job search platform, has good coverage of startups and public companies but I’d still recommend to use LI for network tho
??? Who outside of startups (in a professional environment) even use Slack?
It probably depend on where you live and who you are. LinkedIn is my backup in case of a layoff. It's the site where I can reach everyone who worked with me or have made offers in the past.
If you do what I do, live in my general area and know the right people (which I do), LinkedIn will get you an interview or three lined up in a day or two. None of these people are on Indeed, HackerNews or even Slack.