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No I did not know ! thank you for telling me

You should use Steem logo (3 blue ♨️S), Steemit is only one of the frontends and it's private company. Steem in the blockchain.

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Ah ok I'm going to change that, there is another T-shirt that does not contain the Steemit logo.

Could that be a problem for me ?

The Steemit brand and logo are protected by intellectual property laws, including copyright and other proprietary rights of the United States and other countries. The purpose is to allow Steemit to protect the brand and logo in ways that extend user safety. One may not make unauthorized commercial use of, reproduce, prepare derivative works, distribute copies, perform, or publicly display the Steemit logo or brand, except as permitted by the doctrine of fair use, or as authorized in writing by us.

Section 107 of the Copyright Act provides the statutory framework for determining whether something is a fair use and identifies certain types of uses—such as criticism, comment, news reporting, teaching, scholarship, and research—as examples of activities that may qualify as fair use. Steemit considers artistic variations of the Steemit logo that are used for non-commercial purposes, and are not used to harm Steemit users (i.e. through attraction to phishing sites), an instance of fair use.

The blue Steem logo with the three S-shaped squiggles is licensed under Creative Commons CC0, meaning you’re free to copy, modify, or distribute (even for commercial purposes) without needing to ask permission or give attribution.

(Steemit.com FAQ)

Yes, it could be a problem, because many users won't buy "Steemit" t-shirt, because Steemit is only website. Steem is blockchain with all dApps and Steemit is running on top of Steem.

what can you give me as advice please?

Use word STEEM instead of STEEMIT and this Steem's logo, not Steemit's logo.

I completely changed the T-shirt, to avoid this problem of copyright

Thank you very much for informing me, it's very kind of you