We normally come across posts where it is filtered /avoided to say thanks ,tq , thanks a lot etc.
Why is it that any posts containing thanks are deleted or filtered ?
If so then why don't we raise a flag as its not following the standard procedure here
It's not the case that posts containing "thanks" are deleted. But "thanks" doesn't add to the useful content on the site, so posts that consist only of thanks are deleted because they don't contain useful content. This often happens on questions that attract new users, who post an answer thanking the author of another answer, the same way they might on a web forum or mailing list. Such answers should be flagged as "not an answer" if they don't answer the question, and they'll end up deleted via the "low quality posts" review queue.
Separately, some users write questions as if the question were an email to the mailing list: they include "thanks in advance, Dan" (or whatever their name is) on the bottom the way you would sign-off an email. Again, this is unnecessary on Stack Exchange sites. Your username and profile already show up at the bottom of every question, so this just adds noise. It's not recommended to edit a post just to remove a sign-off, because edits should be substantial and improve multiple issues in the post, but if people are editing anyway, it's commonplace to remove superfluous parts that don't relate to the question. That includes "thanks in advance" as well as common phrases like "help plz" or "urgent help needed".
Stack Exchange sites have a formalised, accepted way of thanking other users for their contributions: upvote helpful questions and answers, and accept the best answer to your question. This allows everyone on the site (who's been here long enough to understand the rules) to thank people without filling every question with hundreds of "thanks" comments. It also means we can count those thanks towards each user's reputation, to make it easier to see which users consistently make helpful additions to the site.
For the particular example you've added to your answer, it sounds like whoever dealt with your flag didn't understand what you were saying, and thought your complaint was that the questioner didn't accept your answer. Be extra clear when raising flags in future.
Custom flags (those raised using the "other" reason) are intended for things that cannot ordinarily be handled by the community. These flags go directly to the moderators, whereas other flags will go into various review queues that can be acted upon by other users of the site. Someone saying "thanks" is simply not something that is generally going to require direct moderator intervention. It is not actively harmful to the site, and there are tools already available to you that are appropriate for the situation:
Secondly, as Dan alludes to in his answer, if you do use the "other" flag reason, you need to be really, excruciatingly specific in your message. Your flag is basically telling us "Hey, something is wrong, and I, as a regular user, am unable to handle it with the current tools. I need a moderator!" Entering your flag description as "saying thanks" doesn't provide us with enough information to act on.
In the context of this flag, you actually raised two separate flags on the same post, one minute apart from each other. The first is the flag you noted above. The second was a flag on one of the comments on the post. Moderators cannot respond to comment flags, so we had to respond to the post flag instead, which appeared to be connected in purpose to your comment flag based on the timestamp and the content of each flag.
