Deleting Retainage Bill that's on Hold

No GL or subledger impact. A retainage bill that's still On Hold hasn't posted anything. No batch, no vendor balance, no hit to the retainage payable account. Deleting it just removes the document.

The part that matters is the original bill. Because the retainage document was never released, the Unreleased Retainage on the source bill is untouched, so the retainage is still sitting there and you can release it correctly later from Release AP Retainage (AP510000). This is the documented fix, not a workaround. See cases 3 and 6 here: https://help.acumatica.com/Wiki/ShowWiki.aspx?pageid=b71b86ba-1317-4681-a182-23d45162b24d

Delete each one from Bills and Adjustments (AP301000) using Delete on the form toolbar.

Worth checking before you start: confirm all 20 really are still unreleased. If Automatically Release Retainage Documents is selected in AP Preferences (AP101000), some of them may have gone to Open on creation, and an Open bill can't be deleted. Those are the ones that need the reversal path in case 7, which is the debit adjustment you're trying to avoid.