Creating a PDF of a PowerPoint document when I don't know if the PPT uses OTF fonts.

You write "after the end of support for TTF" but this is completely wrong, of course there is a lot of misleading stuff on the internet. What you need to know is that it is Type 1 fonts that are going away, NOT TTF (TrueType) fonts. On Windows, these are PFB/PFM files.  

"I've spent an entire day trying to find out whether the fonts in the PPT document are OTF"

Convert the PPT to PDF, open it with Acrobat Pro and go to : Document Properties : Fonts

So sorry that you received such inaccurate information. I'll take a stab to clear up the issues.

whether the fonts in the PPT document are OTF, but either I'm completely stupid or the information isn't available. Adobe Tech Support says Adobe fonts aren't usable in Microsoft documents, but I find that hard to believe.

By @Susan Culligan

First, the issues surrounding "rent a fonts" from Adobe, Linotype, and other font foundries is a ripping hot mess. The system is flaky, at best.

• If the fonts are installed via your subscription to Creative Suite, then they are OpenType/Unicode fonts, either PostScript-flavored or TrueType-flavored. You want to use OpenType/Unicode fonts: they became the computer industry standard in 2000 (23 years ago!!!) and are able to be used cross-platform and cross-technology.
• If you've purchased traditional fonts that you download and install, then you'll have to examine the font's file to determine what technology they are.
  • In Windows, go to C:/Windows/Fonts and right-click on the font, select Properties, and view the information in the Details tab. You also can double-click to open a display of the font's characters. If it says "OpenType" or "Unicode" in the upper left bar, then your fonts are OK.
  • In Mac, use FontBook or your font management program to examine each font. Again, you're looking for the words OpenType or Unicode in the description.
• Adobe's fonts can be embedded into PDFs, and that's a critical option you need when you make a PDF from any program, including PowerPoint.
  • We've found that on Windows, we can easily use Adobe CS fonts in Word and PowerPoint, and export PDFs. (See the attached PDF, which was created in Windows Office 365 and exported with Adobe PDF Maker utility.) 
  • But on Mac it's a different story: the fonts may or may not be available in MS Office/Mac or embeddable into PDFs from Office apps. It's nuts!

Can I create a PDF in 2023 (after the end of support for TTF) from such a document? Please don't ridicule me for this question, I'm at my wit's end 😮

TTF fonts are still supported pretty much everywhere. It's PostScript fonts that are no longer supported in Adobe Creative Suite programs. Remember, the entire computer industry adopted Unicode/OpenType fonts in January 2000...23 years ago. We've all had enough time to read the memo and prepare for this day!

The TTF file extension is used on both OLD TrueType fonts and Unicode/OpenType TrueType fonts, so don't look only at the file extension. Check the font's properties as described above.

We advise our clients and students to avoid using OLD TrueType and PostScript fonts, and use Unicode/OpenType fonts exclusively in your projects. Your files will be more compliant when it's viewed on all kinds of devices and apps and will less likely have missing glyphs or characters.

We also recommend using SIL or open license fonts instead of Adobe's crazy font system. There are a lot of problems with Adobe's fonts licensing and availability. Just search this forum to get a sample of the problems! An excellent option is GoogleFonts, a joint development project of industry bigwigs and professional font designers. Visit https://fonts.google.com

Hope this helps!