It seems to only affect classic theme. When I’ve tried with block theme full site editor (FSE) it was find.
Hey dev team, good luck with that rabbit hole it’s probably and easy one if it’s all modularized.
Side note:
Seems like the deprecated code is the issue. PHP has gone through major updates and it’s probably in the PHP changelog what’s the new replacement for whatever it is that’s deprecated.
Some other programming languages do mention in the trace log what’s the new replacement. I got what if it was ECMAScript JavaScript* or JQuery lib. It’s been awhile and vague where I’ve encountered it.
*There’s the technical committee 39 for ECMAScript aka ECMA TC39