Problem with newly unsupported font

Book Antiqua has been OpenType TT for some time now, and "plain" TT before that. Either work just as well as they ever have.

In any case, they are available with Windows and Office installs. If you are on a Mac, I just checked my Word install and there is a folder with these fonts within.

Can you tell us how you came to the conclusion that your font was no longer supported by InDesign? I suspect that something else is wrong, and that "something else" is causing your font to drop. Did you get a missing font dialog when you opened up the file? 

There are lots of things that can keep a font from loading successfully in InDesign - it doesn't necessarily mean that the font is no longer supported. The one that I personally see most often is the case when someone has a font installed in their Windows\Fonts folder, and also has the same font in a Document Fonts folder in the same folder as their InDesign document. But there are many possible causes, so if you can give us some more details, we might be able to give you some guidance that will let you keep using your Book Antiqua font. 

Hi @alister_6473 , If the font is Type 1 there are font conversion utilities that can convert Type 1 to Open Type. This might help:

https://community.adobe.com/t5/indesign-discussions/ende-der-unterst%C3%BCtzung-f%C3%BCr-ps-type-1-fonts/td-p/11874314

Perhaps @alister_6473 can provide details about font version and font format?

I'm assuming it's a Type 1 font which Adobe has been warning about for quite a few years. I know that doesn't make you feel better but one thing you can try (and which I cannot vouch for) is one of the font converting utilities on the market.

Someone else may come along and have a recommendation or you can search through the community to see if you can find something.