Hello Rabbi,
If I understand Rabbi Azaria Ariel’s answer correctly, the prohibition against using water heated on the holiday by a Sabbath timer is based on the fact that washing one’s entire body with hot water is not something equally relevant to everyone.
Since nowadays washing one’s entire body with hot water is something equally relevant to everyone (that seems obvious to me), should that change the ruling?
Thank you very much

The medieval authorities (Rishonim) disagreed about the reason for the prohibition. Tosafot wrote that it is because it is not something equally relevant to everyone, but in Maimonides and the Rif no reason is given, and it seems that it is because of the bathhouse decree.
It is hard to be lenient regarding water heated on the Sabbath itself. See another overview that also addresses this here:
https://ygolan.org/article-category/%D7%A8%D7%97%D7%99%D7%A6%D7%AA-%D7%9B%D7%9C-%D7%92%D7%95%D7%A4%D7%95-%D7%91%D7%99%D7%95%D7%98/

Thank you very much

Hello Rabbi
I see that in the file you just sent, Rabbi Shmuel is in fact lenient regarding water heated on the holiday by a timer

Well, then if you want, maybe you can rely on him.