10 years back many forums were having even 1M+ members but I am yet to see any modern platforms having such level of engagement.
They ask… on a forum.
blinks slowly
Well at least the spambots dont list us anymore.
Anyway. There was kind of a question asked.
I dont know where you were ever seeing SEO/Webmaster communities with “1M+ members”, tbh, so its really hard to respond. To my knowledge Sitepoint has never had that sort of membership level. Certainly not actively engaged on the forum.
Where is most of the SEO “community” these days? Stroking their e-… with their own blog posts trying to be #1 in the keyword “seospecialistincountyofasodiasd”, as far as i can tell from most of the SEO posts we get around here.
Sorry I rechecked dp forums hit 100,000 figure back then and I was mistaken but point remains the same. I remember even Google employees also used to be part of the forums. I remember Matt Cutts participated in DP forums.
I agree about everybody and their moms have gotten into Guest Posting business wherever I try to go on Facebook groups. It’s just too much.
But they seem to getting good business too. BTW.
I mean… are they? Or are they telling you they are? 
There’s a reason so many new people come around asking the same questions, as many try, and not many succeed… or they set their metric for success at the “but you didnt actually do anything” level. shrug maybe i’m just jaded and there’s a million SEO bloggers out there guest posting and making millions off their businesses. But somehow i doubt it. I just see that whole sphere as an echo chamber where they measure success based on the number of links they get on other blogs in the same sphere… noone really benefits, but everyone “wins”.
Anyway, i’m not sure what you mean by “dp forums”. Never heard of it, or at least not in that shorthand.