I'm trying to delete -iptc:caption-abstract but it refuses to go. I take it it reads from another tag behind the scenes?
No, IPTC:Caption-Abstract is unique. But there are rare cases where it could be a problem to remove. I've encountered it in a few images where some program had written 7 or more separate IPTC blocks. Additionally, the IPTC block can appear in some trailer blocks such as FotoStation (https://exiftool.org/TagNames/FotoStation.html) or AFCP (https://exiftool.org/TagNames/AFCP.html) blocks (normally it's held in the Photoshop block (https://exiftool.org/TagNames/Photoshop.html)).
When I've encountered such files, I've usually had to rewrite all the data as in FAQ #20 (https://exiftool.org/faq.html#Q20). But it's been a long time since I've encountered one.
Just to verify, though, you're reading the IPTC:Caption-Abstract in exiftool after removing? Because some programs will pull data from IPTC:Caption-Abstract, XMP:Description, and EXIF:ImageDescription to fill an entry.