First of all, we should have been informed somehow about these fees and changes to the regulation.
When I gave my consent and accepted the terms and conditions of registration on hitbtc, there was no such problem of inactivity fees. They were introduced later without being announced in any way.
I hate to defend hitbtc but I don't think so. Inactivity fee was actually introduced couple of years ago so if you have been charged for the first time on 10-10-2023, it would mean, the clauses for inactivity fee were already present the moment you registered because accounts are considered inactive after 6 months.
On a different note, hitbtc has been displaying shady practices for years and they remain fine up to this day lol. They have also been ignoring multitude of scam accusations posted about them on different places. So honestly, even if you have a case on this one, it's likely you won't get your money back.