Unfortunately, this particular client was fairly new, so as a result, their Moz campaign data wasn’t around during the high organic traffic times in 2015. However, if it was, a good place to start would be looking at the search visibility metric (as long as the primary keywords have stayed the same). If this metric has changed drastically over the years, it’s a good indicator that your organic rankings have slipped quite a bit.
Regarding Link Detox, links it diagnoses as Toxic are generally fine as they're either not indexed by Google or have malware/viruses/etc., but I recommend a manual review of any diagnosed as Suspicious. I used it recently to get started cleaning up our backlinks and some legit sites and blogs were under Suspicious simply because they didn't have many links pointing to them.
I love your post. I keep coming back because you always have great content I can use in my business as well as share. Since I own my own Digital Marketing company I guess you would be one of THE influencers in Internet Marketing field. I just started my business and because most influencers on twitter are talking about Content Marketing, that is what I have been writing about. But my site is only about a month old so I will just stay consistent in my writing. I’m also in the process of changing my navigation bar so be know how to get to what they want faster. Which would be “what is SEO”, etc. Thanks and would love any advice you can give me.