I recently installed Gnucash, but opening it was surprised to see a list of "recently opened files" that belonged to earlier installations of Gnucash, and that were deleted long ago.

Gnucash FAQs say "The MRU list is now stored in dconf, the replacement for gconf on modern linux systems. You can use a tool called dconf-editor to manipulate the dconf database. You may need to install it using your distribution's software management system first. Then open the DConf Editor gui tool, and use it to modify the /org/gnucash/history tree.
The default number of recently used files shown is set to 4. This can be set to other values by changing the value of maxfiles in the same window."

I installed dconf-editor and found my way to /org/gnucash/history, where I see the obsolete files. If I click one, it is highlighted, but nothing happens if I right-click on it hoping for options, like Delete.
The last line says "maxfiles 4". If I highlight that line and start typing, a field appears showing my entry—I was trying "Maxfiles 1"—but when I hit "enter" the field disappeared and the original entry was unchanged. In any case, this would have been a clumsy solution.

Are there key-commands, or do I need to install something to help dconf-editor to act like an editor?

i cannot help with dconf, but...
1) before re-installing gnucash, have you deleted its old config files?
2) have you tried simply deleting or renaming ~/.local/share/recently-used.xbel?

I started with dconf-editor because that seemed to be what the Gnucash FAQ was suggesting.

Before reinstalling Gnucash I did a file search and deleted everything that I found; I cannot be certain I got everything, but I am disinclined to mess with something that works.

I looked in .../recently-used.xbel but the only mentions I found of Gnucash were my current file.