Would ABS plastic degrade in a dishwasher?
According to several application guides/compatibility charts for ABS plastic (like this one), ABS is fairly resistant to the conditions present in a dishwasher (mild to strong alkaline, salts, no organic solvents, mild temperatures).

Please note that while the ABS structural backbone may be attacked by nucleophilic agents at the nitrile carbon, and that the nitrile group may be hydrolised in acidic or basic environments, the conditions required are much harsher that a common dishwasher.

For example (p.8), glacial acetic acid was found to cause significant swelling, while 25% w/w sodium hydroxide and 25% w/v HCl barely caused changes.

So, while the concern for release of toxic additives present in the formulation of the original ABS filament may or not be significant, from the chemical point of view it is almost sure it won't degrade noticeably.

Also, anecdotally, I used to wash old LEGO pieces in a dishwasher (kids are dirty), and not degradation was observed at all.