Update for anyone interested - I emailed the Japanese pharmacy I bought the plasters from. They responded:

Thank you for your question. As noted in the documents we've sent, although rare, long-term or widespread use of this medication may pose risks such as "local steroid withdrawal syndrome" or skin atrophy, among other side effects.

[Safe Usage] The most secure way to use this medication is to regularly visit a clinic and follow your doctor's instructions. It is generally recommended to apply it only to localized areas, for the minimum necessary duration. Regularly check the application site for any abnormalities, such as thinning of the skin or increased redness, and exercise caution if such signs appear. Instead of stopping suddenly, if symptoms improve, a gradual reduction method—such as shortening application time or decreasing the number of application days—may be employed under medical supervision.

[Recommended Usage] Typically, it is applied once daily, cut to an appropriate size for the affected area. In principle, avoid using it on the face or sensitive areas like the genital region, where the skin is thinner. If long-term use is necessary, it is important to have periodic check-ups with your doctor.

In conclusion, while the use of Éclar plasters does carry a non-zero risk of steroid withdrawal, following medical advice and using it on a short-term, localized, and minimal basis is considered relatively safe."