Of course the use of tags is free in Steem. I mean, anyone is free to use any tag.
Which was the post in which you received that message? Because I check all 3 posts and couldn't find it. Or perhaps it was a private message in the chats?
The @steemstem community also has a discord group here https://discord.gg/9c7pKVD (let me know if the link is working)
In there you can speak with members of the community and consult any doubt you might have.
Thanks for replying, dg. I appreciate your participation.
As it happens, it was a private chat on the discord line.
I invite you to discuss the content of my posts with me - of course I invite any relevant comments and questions about my topics from people here.