Of course reaching out to users is very valuable, it can help you understand them, why they use or stopped using your product, what they like/miss/etc. But to get quality insights like that you will have to ask open questions, interview them, do user tests.
Metrics tell you if something works or not, reaching out to users helps you understand why it does or doesn't.
A) Identify your users, who they are?
B) Find active niche places where they are in high density. These could be specific people on LN you can find searching by role/position, topic-oriented FB groups, industry-specific conferences and events.
C) Share your product hypothesis and ask for their feedback.
Specifically, do not expect a spot-on feedback from people who are not in your target group - indiehackers or HN are nice places full of great people, but are they really your potential clients? Or should you rather look elsewhere.