@defaultgfg6f7neewdw,
To answer your first question, Adobe never, repeat never, repeat yet again never removes any fonts that you have installed on your system, whether those fonts are bundled with your operating system (in this case MacOS 11) or Microsoft Office or any other application. Thus, if you can't find a font that you need, it is not because Adobe removed it.
Secondly, Apple does not bundle the Bookman Old Syle font family with MacOS. They never did!
At one time Microsoft Office applications under MacOS installed a number of font families including the Bookman Old Style font family on your computer. Recent versions of Microsoft Office for Mac no longer do such font installation. Rather, they do provide access to that family and many other fonts only to Microsoft Office for Mac applications while you are running them. The fonts are actually located as resources within the Office applications themselves and are not externally visible. (Actually, there are 264 distinct fonts hidden by Microsoft in this manner!) I cannot vouch for this, but it is possible that updates from older versions of Office to newer versions of Office on Mac may uninstall those fonts that were previously installed on your system; if I recall correctly, those fonts were installed on the system by Microsoft a few years back!
Thus, there is no kosher method of accessing the Bookman Old Style family (and the other fonts of those hidden 264 fonts) outside of Microsoft Office for Mac unless you separately license and install those fonts directly on your system.
Again, this is absolutely not an Adobe issue!!!!!