Using custom skins on official servers by modifying game files has always been forbidden since CS:GO. CS2 doesn't make it worse. Join a community server if you want to use your own skin you made up.

At one time[1] Valve even tried to ban community servers that offer custom models/skins. Though they doesn't seem to enforce it anymore.

[1]: https://blog.counter-strike.net/index.php/server_guidelines/