@RanWest thanks for the heads up. That page is in work in progress mode as of now - I had to take down my site a while ago for some issues and I am taking the occasion to rebrand it and rewrite articles and pages in a better way (I think I caused the site issue with my attempts at cooking something, ClassicPress was working fine, it was my own code that broke the whole thing but having the occasion to improve what I wrote is not a bad thing). It’s content will be updated and rebuilt completely (to include a more in depth reasoning on the why basically because on second thought I feel it was not really exaustive) and I have decided meanwhile to avoid experimenting on my main live site (that is a bad habit of mine, everytime I mess up I promise I won’t do it anymore but after some time I go back to doing it - and I do have local sites and also test staging sites).
EDITED TO ADD:
Also the deprecation is due to a theme I am testing to see if I want to port it to CP. There is a function that CP v2 deprecated that WP themes do use that is the add_support() for HTML5 one, since CP does not need it it means it throws deprecations BUT theme still does work. One can delete it from the functions.php file of the theme and the theme continues working fine OR use a code that Simone shared on this forum (search for HTML5 to find the thread) that modifies it to be a conditional check (if theme is used in CP skip the function, if it’s used in WP use it).
I like this specific theme layout and customization options, however in the backend for its theme page it uses thickbox - this means that to port to CP I will need to clean it up from blocks/FSE stuff, modify the thickbox code and remove/modify the HTML5 support function and in doing so maintain it WPCS/CPCS and PHP 8.4 compatible at least.
EDITED TO ADD 2:
To be complete, I went to search for the code Simone shared for themes add_support(HTML5) function, you can read about it here.