How was this handled before Ledger 90A was in use? Waiting for over a year to enter and post adjusting entries? Did they have to keep separate books for each fiscal year?

If their business is always in catch-up mode and having to wait a year for adjusting entries, perhaps they will have to forgo the multiple years of history feature in Ledger 9, and instead keep each year in separate databases to allow posting to whichever year is required. Then use Adagio FX to consolidate the amounts for financial reporting.

Or do not open the Next Year and instead Archive the batches that belong there until the Prior year entries are complete. Then create the Next Year and copy the Archive batches to Next Year, recalculating fiscal periods as required to overwrite period 99. (sound familiar from a past era...?)

It would be a very substantial change to allow posting to a prior year when the next year is open - having to do 2 year ends in the background to accommodate both the prior year and current year accounts and retained earnings as well as next year. And many other structural considerations.