This is my personal opinion
I think posting a thank you with message na may mga insights or other comments sa mga thread na very helpful (informative,constructive) ay para saakin hindi ko cinoconsider as a spam, May mga limitation din for me like posting a "thank you" post sa mga thread na matagal na nag eexist(necro bumping) ay for me considered as spam. We all know that this is a forum and every comments/post is allowed as long as hindi nalalabag ang forum rules. Even though na alam naman natin na may mga bounty manager dito na hindi nag hindi inaallow/binibilang sa kanilang signature campaign ang mga short post (150 characters below kadalasan) as there safety measure na maging constructive ang post ng kanilang bounty members.
My conclusion is there is some limitation on giving thank you on a post, For me hindi ito spam pero dapat nilulugar ang pag papasalamat. As mentioned above its better to give them merit because its a way of giving thanks sa mga informative and constructive post.
I'm sorry kung namention na ang ibang content ng post ko pero I need to include that kasi it is my own opinion